The term adult has at least three distinct meanings. It can indicate a biologically grown or mature person. It may also mean a plant, animal, or person who has reached full growth or alternatively is capable of reproduction, or the classification legal adult, generally determined as a person who has attained the legally fixed age of majority; as opposed to a minor.
Adulthood can be defined in biology, psychological adult development, law, personal character, or social status. These different aspects of adulthood are often inconsistent and contradictory. A person may be biologically an adult, and have adult behavior but still be treated as a child if they are under the legal age of majority. Conversely one may legally be an adult but possess none of the maturity and responsibility that define adult character.
Coming of age is an event; passing a series of tests to demonstrate the child is prepared for adulthood; or reaching a specified age, sometimes in conjunction with demonstrating preparation. Most modern societies determine legal adulthood based on reaching a legally-specified age without requiring a demonstration of physical maturity or preparation for adulthood.
Some propose that moving into adulthood involves an emotional structuring of denial, suggesting this process becomes necessary to cope with one's own behavior, especially in uncomfortable situations, and also the behavior of others.
Historically and cross-culturally, adulthood has been determined primarily by the start of puberty (the appearance of secondary sex characteristics such as menstruation in women, ejaculation in men, and pubic hair in both sexes). In the past a person usually moved from the status of child directly to the status of adult, often with this shift being marked by some type of coming-of-age test or ceremony.
Most medical and general English dictionaries define childhood as the period from infancy to puberty, thus historically adulthood began with puberty. After the social construct of adolescence was created, adulthood split into two forms: social adulthood and biological adulthood. Thus, there are now two primary forms of adults: biological adults (people who have attained reproductive ability, are fertile, or who evidence secondary sex characteristics) and social adults (people who are recognized by their culture and/or law as being adults). Depending on the context, adult can indicate either definition.
Although few or no established dictionaries provide a definition for the two word term biological adult, the first definition of adult in multiple dictionaries  includes "the stage of the life cycle of an animal after reproductive capacity has been attained". Thus, the base definition of the word adult is the period beginning at puberty. Although this is the primary definition of the base word adult, the two word term biological adult stresses or clarifies that the original definition, based on the beginning of puberty, is being used (that is, the organism has matured to the biologically important point of being able to reproduce).
Although there is no scientific agreement on when physical maturation completes, in modern society social adulthood somewhat corresponds to the completion of physical maturation. Because the term adult is most often used without the adjective social or biological, and since the term is frequently used to refer to social adults, some writers have mistaken the meaning of the two word phrase biological adult to begin at the end of physical maturation rather than the onset of puberty.
In modern developed countries, puberty generally begins around 10 years of age for girls and 12 years of age for boys, though this will vary from person to person. The age of an adult is 20-
Legally it means that one can engage in a contract. The same or a different minimum age may be applicable to, for example, parents losing parenting rights and duties regarding the person concerned, parents losing financial responsibility, marriage, voting, having a job, serving in the military, buying/possessing firearms (if legal at all), driving, traveling abroad, involvement with alcoholic beverages (if legal at all), smoking, sex, gambling (both lottery and casino) being a prostitute or a client of a prostitute (if legal at all), being a model or actor in pornography, etc. Admission of a young person to a place may be restricted because of danger for that person, concern that the place may lead the person to immoral behavior, and/or because of the risk that the young person causes damage (for example, at an exhibition of fragile items).
One can distinguish the legality of acts of a young person, and of enabling a young person to carry out that act, by selling, renting out, showing, permitting entrance, participating, etc. There may be distinction between commercially and socially enabling. Sometimes there is the requirement of supervision by a legal guardian, or just by an adult. Sometimes there is no requirement, but just a recommendation.
With regard to pornography one can distinguish:
With regard to films with violence, etc.:
The legal definition of entering adulthood usually varies between ages 15–21, depending on the region in question. Some cultures in Africa define adult at age 13.
According to Jewish tradition, adulthood is reached at age 13 (the age of the Bar Mitzvah), for Jewish boys, for example, were expected to demonstrate preparation for adulthood by learning the Torah and other Jewish practices. The Christian Bible and Jewish scripture has no age requirement for adulthood or marrying, which includes engaging in sexual activity. The age of consent for sexual relations in the Vatican is 13, the age when many girls achieve menses, and are thus reproductive and therefore physically capable of marriage and childbirth. However, the Vatican does not allow sex outside of marriage. The 1983 Code of Canon Law states, "A man before he has completed his sixteenth year of age, and likewise a woman before she has completed her fourteenth year of age, cannot enter a valid marriage". According to The Disappearance of Childhood by Neil Postman, the Christian Church of the Middle Ages considered the age of accountability, when a person could be tried and even executed as an adult, to be age 7.
In most of the world, including most of the United States, parts of the United Kingdom (England, Northern Ireland, Wales), India and China, the legal adult age is 18 (historically 21) for most purposes, with some notable exceptions:
Adult video games are video games which have significant sexual content (like adult movies), and are therefore intended for an adult audience. Adult games can fall into many genres and have diverse gameplay.
In console gaming, the genre can be seen as early as the Atari 2600, such as Custer's Revenge. In PCs, the genre can be first seen in adult text adventures, and later with graphical adventures, including one of the most recognized and mainstream adult adventure game series, Leisure Suit Larry. Arcade games have received few adult entries in North America.
With CD-ROM and multimedia based games in the 1990s, most adult games featured video clips with limited interactivity. Both pre-rendered and realtime 3D graphics were also used. While most games could be considered nothing more than pornography, some attempted to include actual story and plot. This can be seen in some games with less explicit content, equal to an R or PG-13 rated movie.
Modern console publishers often have policies against depictions of nudity and explicit sexuality, particularly Sony with its PlayStation brand of consoles. Some titles featuring nudity, such as BMX XXX and Leisure Suit Larry: Magna Cum Laude for the PlayStation 2 were censored, while versions on the Xbox and GameCube were not. However, Sony allowed nudity in the title God of War, which was also developed by Sony.
A new generation of adult social games has emerged that bring multiple users together in sexual environments. Examples include Red Light Center, Singles: Flirt Up Your Life and Playboy: The Mansion. While it is not explicitly intended for purely adult-oriented entertainment, the virtual world of Second Life, which is made up almost entirely of player-made content, has a very exotic array of adult entertainment including nudity and full-on sexual activities.
Adult games may take the form of bootlegs, circumventing mainstream publishers who may have policies against such games. Patches or hacks to mainstream non-adult games may add sexual and pornographic themes, mostly for humor, especially when sexuality was never intended in the original game. Examples include the Tomb Raider computer games, Grand Theft Auto: San Andreas' Hot Coffee mod, The Elder Scrolls IV: Oblivion (which has multiple such mods) and ROM hacks for console emulators. There is also a mod for World of Warcraft showing nude features.
The Internet has allowed adult games to receive wider availability and recognition, including amateur games in Flash or Java. It has also allowed amateurs to create and distribute adult text adventure games, known as "Adult Interactive Fiction" or AIF.
Japanese eroge (エロゲー erogē ?), also known as hentai games, have their origins in the 1980s, when Japanese companies introduced their own brands of microcomputer to compete with those of the United States. Competing systems included the Sharp X1, Fujitsu FM-7, MSX, and NEC PC-8801. NEC was behind its competitors in terms of hardware (with only 16 colors and no sound support) and needed a way to regain control of the market. Thus came the erotic game. Early eroge had simple stories, often involving rape. It made the PC-8801 popular, but customers quickly tired of paying 8800 yen ($85) for such simple games. Soon, new genres were invented: ASCII's Chaos Angels, a role-playing-based eroge, inspired Dragon Knight by Elf and Rance by AliceSoft.
In 1992, Elf released Dōkyūsei. In it, before any eroticism, the user has to first win the affection of one of a number of female characters, making the story into an interactive romance novel. Thus, the love simulation genre was invented. Soon afterwards, the video game Otogirisou on the Super Famicom attracted the attention of many Japanese gamers. Otogirisou was a standard adventure game but had multiple endings. This concept was called a "sound novel."
In 1996, the new software publisher Leaf expanded on this idea, calling it a visual novel and releasing their first successful game, Shizuku, a horror story starring a rapist high school student, with very highly reviewed writing and music. Their next game, Kizuato, was almost as dark. However, in 1997, they released To Heart, a sweetly sentimental story of high school love that became one of the most famous and trendsetting eroge ever. To Heart's music was so popular it was added to karaoke machines throughout Japan — a first for eroge.
After a similar game by Tactics, One: Kagayaku Kisetsu e, became a hit in 1998, Visual Art's scouted main creative staff of One to form a new brand under them, which became Key. In 1999, Key released Kanon. It contains only about 7 brief erotic scenes in a sentimental story the size of a long novel (an all-ages version was also released afterward), but the enthusiasm of the response was unprecedented, and Kanon sold over 300,000 copies. In 2002 a 13-episode anime series was produced, as well as another 24-episode anime series in 2006. According to Satoshi TODOME's A History of Eroge, Kanon is still the standard for modern eroge and is referred to as a "baptism" for young otakus in Japan. Although many eroge still market themselves primarily on sex, eroge that focus on story are now a major established part of Japanese otaku culture.