A prefix is a part of a word. It is put at the start of another word to make a different meaning. It can also mean a number that is put in at the start to show which number something is in a group. It is a type of affix.

• Grandfather means your parent's father.
• Wineglass is a glass that you put wine in.

Strictly speaking these constructions do not use prefixes, but make compound nouns (in other cases also compound adjectives).

The following examples illustrate real prefixes.

Often people want to use a prefix to mean not. There are many prefixes that mean not.

• Unnatural means not natural.
• Apolitical means not political.
• Nonviolent means not violent.

Other examples of prefixes:

• bicentennial (and other number related prefixes: mono, ter/tri, tetra, quadro, penta, hepta, octo, nano etc.)
• contradiction
• encompass
• paratrooper
• percent
• periscope
• prototype
• polytechnical
• react
• semicolon

Scientists and doctors use prefixes in many words.

• Hepato- is a prefix that means liver. So hepatocellular means 'about liver cells'.
• Hydro- is a prefix that means water. So Hydroelectric dams are dams that make electricity from the flow of water.
• Pre- or Ante- are prefixes that mean before. Prenatal diagnostics is the diagnostics done before the birth of a baby.
• Post- is a prefix that means after. Post-traumatic experience would be an experience that was made after trauma.

Sometimes people make up words by adding a prefix. These words are not in a dictionary. But if people use these words enough, sometimes they go into dictionaries. For example, we can make the word unsimple, which splits up into not simple. This is not a word in a dictionary.

Compare to suffix. Suffixes are letters put at the end of a word to change its meaning.

the prefixs, can be also: in and re