A synthetic language is one with a high morpheme-to-word ratio. This linguistic classification is largely independent of morpheme-usage classifications (such as inflectional, agglutinative, etc.) but there is a common tendency for agglutinative languages to additionally be synthetic.

Synthetic languages are often contrasted with isolating languages, however it is often better to consider isolating and synthetic as existing on a continuum, with strictly isolating (consistently one morpheme per word) at one end and highly polysynthetic (where one word may contain the equivalent of an English sentence) at the other. Synthetic languages would thus be placed at the median point of this scale, and thus it should be clear that there is no hard and fast boundary between these classifications.

There are numerous examples of synthetic languages, the most commonly cited being Indo-European languages such as German and Russian, virtually all of the Altaic language family (Turkish, Mongolian and many Siberian languages), the Uralic languages (Finnish, Estonian, Hungarian), Japanese, Korean, as well as many Native American languages including Navajo, Nahuatl, Iroquoian languages (Mohawk, Seneca, Mingo although see polysynthesis), Quechua and languages of the Canadian Northwest Coast (Kwakiutl) and the Amazon rain forest (Desana).

Types of synthesis

There are several ways in which a language can exhibit synthetic characteristics:

1. Derivational synthesis: Morphemes of different types (nouns, verbs, derivational prefixes or suffixes, etc.) are joined to create new words. For example:

German: Luftkissenfahrzeug "air-cushion-travel-machine" = "hovercraft"

(Sino-)Japanese: teishaeki "stop-car-station" = "station where the train stops"

Finnish: pikakaurahiutaleannos "quick-oat-flake-ration" = "a serving of quick oatmeal"

English: indisputably = "not-dispute-possible-ADVERB"

1. Relational synthesis: Roots are joined to bound morphemes to show grammatical functions:

Nahuatl: ocaltizquiya "already-(she)-him-bathe-would" = "she would have bathed him"

Japanese: miseraregatai "see-causative-passive-difficult" = "it"s difficult to be shown (this)"

Finnish: juokse·ntel·isi·n·ko·han "run-erratic-potential-I-question-casual" = "I wonder if I should run around (aimlessly)"

Degrees of synthesis

In order to demonstrate the "continuum" nature of the isolating-synthetic-polysynthetic classification, some examples are shown below:

• Strictly isolating - Tahitian: Ua marere te manu na te ara = "The bird flew off into the distance". Virtually every word is a stand-alone morpheme.
• Rather isolating - English: "He travelled by hovercraft on the sea". Largely isolating, but travelled and hovercraft each have two morphemes per word, the former being an example of relational synthesis (inflection), and the latter of derivational synthesis (derivation).
• Rather synthetic - Japanese: Watashitachi ni totte, kono naku kodomo no shashin wa miseraregatai mono desu = "For us, it's difficult to be shown these pictures of children crying". Virtually every word has more than one morpheme and some have up to five (the particles ni, no, wa are enclitic, i. e. phonologically part of the previous word).
• Very synthetic - Finnish: Käyttäytyessään tottelemattomasti oppilas saa jälki-istuntoa = "Should he behave in an insubordinate manner, the student will get detention.". Practically every word is derived and/or inflected, and one word can be considered polysynthetic.
• Polysynthetic - Mohawk: Washakotya'tawitsherahetkvhta'se = "He ruined her dress" (strictly, "He made the thing that one puts on one's body ugly for her"). One word expresses the idea that would be conveyed in an entire sentence in a non-polysynthetic language.

oligosynthesis

• Oligosynthetic languages are a theoretical notion created by Benjamin Whorf with no known examples existing in natural languages. Functionally synthetic, but makes use of a very limited array of morphemes (perhaps just a few hundred). Whorf proposed that Nahuatl was oligosynthetic. This has since been discounted by most linguists.