In linguistics, a compound is a lexeme (less precisely, a word) that consists of more than one stem. Compounding or composition is the word-formation that creates compound lexemes (the other word-formation process being derivation). Compounding or Word-compounding refers to the faculty and device of language to form new words by combining or putting together old words. In other words, compound, compounding or word-compounding occurs when a person attaches two or more words together to make them one word. The meanings of the words interrelate in such a way that a new meaning comes out which is very different from the meanings of the words in isolation.
Compound formation rules vary widely across language types.
In a synthetic language, the relationship between the elements of a compound may be marked with a case or other morpheme. For example, the German compound Kapitänspatent consists of the lexemes Kapitän (sea captain) and Patent (license) joined by an -s- (originally a genitive case suffix); and similarly, the Latin lexeme paterfamilias contains the (archaic) genitive form familias of the lexeme familia (family). Conversely, in the Hebrew language compound, the word בֵּית סֵפֶר bet sefer (school), it is the head that is modified: the compound literally means "house-of book", with בַּיִת bayit (house) having entered the construct state to become בֵּית bet (house-of). This latter pattern is common throughout the Semitic languages, though in some it is combined with an explicit genitive case, so that both parts of the compound are marked.
Agglutinative languages tend to create very long words with derivational morphemes. Compounds may or may not require the use of derivational morphemes also. The longest compounds in the world may be found in the Finnish and Germanic languages. In German, extremely long compound words can be found in the language of chemical compounds, where in the cases of biochemistry and polymers, they can be practically unlimited in length. German examples include Farbfernsehgerät (color television set), Funkfernbedienung (radio remote control), and the jocular word Donaudampfschifffahrtsgesellschaftskapitänsmütze (Danube steamboat shipping company Captain's hat).
In Finnish there is no theoretical limit to the length of compound words, but in practice words consisting of more than three components are rare. Even those can look mysterious to non-Finnish, take hätäuloskäytävä (emergency exit) as an example. Internet folklore sometimes suggests that lentokonesuihkuturbiinimoottoriapumekaanikkoaliupseerioppilas would be the longest word in Finnish, but evidence of it actually being used is scant and anecdotic at best.
Compounds can be rather long when translating technical documents from English to some other language, for example, Swedish. "Motion estimation search range settings" can be directly translated to rörelseuppskattningssökintervallsinställningar; the length of the words are theoretically unlimited, especially in chemical terminology.
A common semantic classification of compounds yields four types:
An endocentric compound consists of a head, i.e. the categorical part that contains the basic meaning of the whole compound, and modifiers, which restrict this meaning. For example, the English compound doghouse, where house is the head and dog is the modifier, is understood as a house intended for a dog. Endocentric compounds tend to be of the same part of speech (word class) as their head, as in the case of doghouse. (Such compounds were called tatpuruṣa in the Sanskrit tradition.)
Exocentric compounds (called a bahuvrihi compound in the Sanskrit tradition) do not have a head, and their meaning often cannot be transparently guessed from its constituent parts. For example, the English compound white-collar is neither a kind of collar nor a white thing. In an exocentric compound, the word class is determined lexically, disregarding the class of the constituents. For example, a must-have is not a verb but a noun. The meaning of this type of compound can be glossed as "(one) whose B is A", where B is the second element of the compound and A the first. A bahuvrihi compound is one whose nature is expressed by neither of the words: thus a white-collar person is neither white nor a collar (the collar's colour is a metaphor for socioeconomic status). Other English examples include barefoot and Blackbeard.
Copulative compounds are compounds which have two semantic heads.
Appositional compounds refer to lexemes that have two (contrary) attributes which classify the compound.
|endocentric||A+B denotes a special kind of B||darkroom, smalltalk|
|exocentric||A+B denotes a special kind of an unexpressed semantic head||skinhead, paleface (head: 'person')|
|copulative||A+B denotes 'the sum' of what A and B denote||bittersweet, sleepwalk|
|appositional||A and B provide different descriptions for the same referent||actor-director, maidservant|
Most natural languages have compound nouns. The positioning of the words (i. e. the most common order of constituents in phrases where nouns are modified by adjectives, by possessors, by other nouns, etc) varies according to the language. While Germanic languages, for example, are left-branching when it comes to noun phrases (the modifiers come before the head), the Romance languages are usually right-branching.
In French, compound nouns are often formed by left-hand heads with prepositional components inserted before the modifier, as in chemin-de-fer 'railway' lit. 'road of iron' and moulin à vent 'windmill', lit. 'mill (that works)-by-means-of wind'.
In Turkish, one way of forming compound nouns is as follows: yeldeğirmeni ‘windmill’ (yel: wind, değirmen-i:mill-possessive); demiryolu 'railway'(demir: iron, yol-u: road-possessive).
A type of compound that is fairly common in the Indo-European languages is formed of a verb and its object, and in effect transforms a simple verbal clause into a noun.
In Spanish, for example, such compounds consist of a verb conjugated for third person singular, present tense, indicative mood followed by a noun (usually plural): e.g., rascacielos (modelled on "skyscraper", lit. 'scratches skies'), sacacorchos 'corkscrew', lit. 'removes corks'). These compounds are formally invariable in the plural (but in many cases they have been reanalyzed as plural forms, and a singular form has appeared). French and Italian have these same compounds with the noun in the singular form: Italian grattacielo 'skyscraper', French grille-pain 'toaster', lit. 'toasts bread', and torche-cul 'ass-wipe' (Rabelais: See his "propos torcheculatifs").
This construction exists in English, generally with the verb and noun both in uninflected form: examples are spoilsport, killjoy, breakfast, cutthroat, pickpocket, dreadnought, and know-nothing.
Also common in English is another type of verb-noun (or noun-verb) compound, in which an argument of the verb is incorporated into the verb, which is then usually turned into a gerund, such as breastfeeding, finger-pointing, etc. The noun is often an instrumental complement. From these gerunds new verbs can be made: (a mother) breastfeeds (a child) and from them new compounds mother-child breastfeeding, etc.
In the Australian Aboriginal language Jingulu, (a Pama-Nyungan language), it is claimed that all verbs are V+N compounds, such as "do a sleep", or "run a dive", and the language has only three basic verbs: do, make, and run.
A special kind of composition is incorporation, of which noun incorporation into a verbal root (as in English backstabbing, breastfeed, etc.) is most prevalent (see below).
Verb-verb compounds are sequences of more than one verb acting together to determine clause structure. They have two types:
Serial verb expressions in English may include What did you go and do that for?, or He just upped and left; this is however not quite a true compound since they are connected by a conjunction and the second missing arguments may be taken as a case of ellipsis.
Compound prepositions formed by prepositions and nouns are common in English and the Romance languages (consider English on top of, Spanish encima de, etc.). Japanese shows the same pattern, except the word order is the opposite (with postpositions): no naka (lit. "of inside", i.e. "on the inside of"). Hindi has a small number of simple (i.e., one-word) postpositions and a large number of compound postpositions, mostly consisting of simple postposition ke followed by a specific postposition (e.g., ke pas, "near"; ke nīche, "underneath").
In the Russian language compounding is a common type of word formation, and several types of compounds exist, both in terms of compounded parts of speech and of the way of the formation of a compound. 
Compound nouns may be agglutinative compounds, hyphenated compounds (stol-kniga 'folded table' lit. 'table-book', i.e., "book-like table"), or abbreviated compounds (portmanteaux: kolkhoz). Some compounds look like portmanteaux, while in fact they are an agglutinations of type stem + word: "Akademgorodok" (from "akademichesky gorodok" 'Academic Townlet', i.e., Academic Village). In agglutinative compound nouns, an agglutinating infix is typically used: parokhod 'steamship': par + o + khod. Compound nouns may be created as noun+noun, adjective+noun, noun+adjective (rare), noun+verb (or, rather, noun+verbal noun).
Compound adjectives may be formed either per se, e.g., "belo-rozovy" 'white-pink' or as a result of compounding during the derivation of an adjective from a multiword term: Каменноостровский проспект ([kəmʲɪnnʌʌˈstrovskʲɪj prʌˈspʲɛkt]) 'Stone Island Avenue', a street in St.Petersburg.
Reduplication in Russian language is also a source of compounds.
In Germanic languages, compound words are formed by prepending a descriptive word in front of the main word. A good example is "starfish"; it is a "fish" that is in the shape of a"star". Each part may in turn be a compound word, so there is no problem making an arbitrary long word. This contrasts to Romance languages, where prepositions are more used to specify such word relationships instead of concatenating the words.
As a member of the Germanic family of languages, English is special in that compound words are usually written by separating them into their parts. Although English does not form compound nouns to the extent of Dutch or German, such constructions as "Girl Scout troop", "city council member", and "cellar door" are arguably compound nouns and used as such in speech. Writing them as separate words is merely an orthographic convention, possibly a result of influence from French.
Although there is no universally agreed-upon guideline regarding the use of compound words in the English language, in recent decades written English has displayed a noticeable trend towards increased use of compounds. Recently, many words have been made by taking syllables of words and compounding them, such as pixel (picture element) and bit (binary digit). This is called a syllabic abbreviation. Moreover, the English way of spelling compound words is spreading to other languages: There is a trend in Scandinavian languages towards splitting compound words, known in Norwegian as "orddelingsfeil" (word split error). Because the Norwegian language relies heavily on the distinction between the compound word and the sequence of the separate words it consists of, this has dangerous implications. For example "røykfritt" (smokefree, meaning no smoking) has been seen confused with "røyk fritt" (smoke freely).