compound

In linguistics, a compound is a lexeme (less precisely, a word or sign) that consists of more than one stem. Compounding, composition or nominal composition is the process of word formation that creates compound lexemes. Compounding occurs when two or more words or signs are joined to make a longer word or sign.
The meaning of the compound may be similar to or different from the meaning of its components in isolation. The component stems of a compound may be of the same part of speech—as in the case of the English word footpath, composed of the two nouns foot and path—or they may belong to different parts of speech, as in the case of the English word blackbird, composed of the adjective black and the noun bird. With very few exceptions, English compound words are stressed on their first component stem.
As a member of the Germanic family of languages, English is unusual in that even simple compounds made since the 18th century tend to be written in separate parts. This would be an error in other Germanic languages such as Norwegian, Swedish, Danish, German and Dutch. However, this is merely an orthographic convention: As in other Germanic languages, arbitrary noun phrases, for example "girl scout troop", "city council member", and "cellar door", can be made up on the spot and used as compound nouns in English too.
The process occurs readily in all Germanic languages for different reasons. Words can be concatenated both to mean the same as the sum of two words (e.g. Pressekonferenz—German for press conference) or where an adjective and noun are compounded (e.g. hvidvinsglas—Danish for white wine glass). This can create a plethora of large, but valid words in these languages, by compounding compound words with several more.
The addition of affix morphemes to words (such as suffixes or prefixes, as in employ → employment) should not be confused with nominal composition, as this is actually morphological derivation.
Some languages easily form compounds from what in other languages would be a multi-word expression. This can result in unusually long words, a phenomenon known in German (which is one such language) as Bandwurmwörter or tapeworm words.
Sign languages also have compounds. They are created by combining two or more sign stems.
So-called "classical compounds" are compounds derived from classical Latin or ancient Greek roots.

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