Polysynthetic languages are highly
synthetic languages, i.e. languages in which words are composed of many
morphemes.
The degree of synthesis refers to the morpheme-to-word ratio. Languages with more than one morpheme per word are synthetic. Polysynthetic languages lie at the extreme end of the synthesis continuum with a very high number of morphemes per word (at the other extreme are isolating or analytic languages with only one morpheme per word). These highly synthetic languages often have very long words that correspond to complete sentences in less synthetic languages.
Many, if not most, languages regarded as polysynthetic include agreement with object arguments as well as subject arguments in verbs. Incorporation (primarily noun incorporation) has been an issue that has historically been confused with polysynthesis and also used as a criterion for its definition. Incorporation refers to the phenomenon where lexical morphemes (or lexemes) are combined together to form a single word. Not all polysynthetic languages are incorporating, and not all incorporating languages are polysynthetic.
A contrast was made by some linguists between oligosynthetic and polysynthetic languages, where the former term was applied to languages with relatively few morphemes. The distinction is not widely used today.
