An agglutinative language is a type of synthetic language with morphology that primarily uses agglutination. Words may contain different to determine their meanings, but all of these morphemes (including word stem and ) remain, in every aspect, unchanged after their unions. This results in generally more easily deducible word meanings if compared to fusional languages, which allow modifications in either or both the phonetics or spelling of one or more morphemes within a word, usually shortening the word or providing easier pronunciation. Agglutinative languages have generally one grammatical category per affix while fusional languages have multiple. The term was introduced by Wilhelm von Humboldt to classify languages from a morphological point of view.
Non-agglutinative synthetic languages are fusional languages; morphologically, they combine affixes by "squeezing" them together, drastically changing them in the process, and joining several meanings in a single affix (for example, in the Spanish language word comí "I ate", the suffix - í carries the meanings of first person, singular number, past tense, perfective aspect, indicative mood, active voice.) The term agglutinative is sometimes incorrectly used as a synonym for synthetic. Used in this way, the term embraces both fusional languages and inflected languages. The agglutinative and fusional languages are two ends of a continuum, with various languages falling more toward one or the other end. For example, Japanese is generally agglutinative, but displays fusion in younger brother, from oto+hito (originally woto+pito), and in its non-affixing verb conjugations. A synthetic language may use morphological agglutination combined with partial usage of fusional features, for example in its case system (e.g., German language, Dutch language, and Persian language).
Agglutinative languages tend to have a high rate of affixes or morphemes per word, and to be very regular, in particular with very few irregular verbs. For example, Japanese has very few irregular verbs – only two are significantly irregular, and there are only about a dozen others with only minor irregularity; Luganda has only one (or two, depending on how "irregular" is defined); while in the Quechua languages, all the ordinary verbs are regular. In Turkish language, there is only one irregular noun (, meaning water), no irregular verbs other than the copular verbs, and two existential particles. Korean language has only ten irregular forms of conjugation except passive and causative conjugations. Georgian is an exception; it is highly agglutinative (with up to eight morphemes per word), but it has a significant number of irregular verbs with varying degrees of irregularity.
Many languages spoken by Ancient Near East peoples were agglutinative:
Agglutination is a typological feature and does not imply a linguistic relation, but there are some families of agglutinative languages. For example, the Proto-Uralic language, the ancestor of the Uralic languages, was agglutinative, and most descended languages inherit this feature. But since agglutination can arise in languages that previously had a non-agglutinative typology and it can be lost in languages that previously were agglutinative, agglutination as a typological trait cannot be used as evidence of a genetic relationship to other agglutinative languages. The uncertain theory about Ural-Altaic proffers that there is a genetic relationship with this proto-language as seen in Finnish language, Mongolian and Turkish language. Nicholas Poppe, The Uralo-Altaic Theory in the Light of the Soviet Linguistics Accessed 2010-04-07
Many languages have developed agglutination. This developmental phenomenon is known as language drift. There seems to exist a preferred evolutionary direction from agglutinative synthetic languages to fusional synthetic languages, and then to non-synthetic languages, which in their turn evolve into isolating languages and from there again into agglutinative synthetic languages. However, this is just a trend, and in itself a combination of the trend observable in Grammaticalization theory and that of general linguistic attrition, especially word-final apocope and elision.