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 ) tend to remain unchanged after their unions, although this is not a rule: for example, Finnish language is a typical agglutinative language, but morphemes are subject to (sometimes unpredictable) consonant alternations called consonant gradation, and the so-called "ta" and "te" forms of Japanese (also an agglutinative language) fuse with the stem in regular but phonetically unpredictable ways (for example the "te-form" of 切る kiru is 切って kitte and not * kiru-te) . Despite those occasional alternations, agglutinative languages tend to have more easily deducible word meanings if compared to fusional languages, which allow unpredictable modifications in either or both the phonetics or spelling of one or more morphemes within a word. This usually results in a shortening of the word, or it provides easier pronunciation.
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, but that term also includes fusional 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 some nouns, such as extra="younger brother", from oto + hito (originally woto + pito, "young, younger" + "person"), and Japanese verbs, adjectives, the copula, and their affixes undergo sound transformations. For example, extra="to write; someone writes" affixed with extra=politeness suffix and extra=past tense marker becomes extra="someone wrote", with the -mas- portion used to express a politely distanced social context to the intended audience. 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. Both Georgian and Korean language are exceptions; such languages have a significant number of irregular verbs.
Many unrelated languages spoken by Ancient Near East peoples were agglutinative, though none from larger families have been identified:
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.