An agglutinative language is a type of synthetic language with morphology that primarily uses agglutination. Words may contain different morphemes to determine their meanings, but all of these morphemes (including stems and affixes) tend to remain unchanged after their unions, although this is not a rule: for example, Finnish is a typical agglutinative language, but morphemes are subject to (sometimes unpredictable) consonant alternations called consonant gradation. Despite the occasional outliers, 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.
Agglutinative languages have generally one per affix while fusional languages have multiple. The term was introduced by Wilhelm von Humboldt to classify languages from a morphological point of view. It is derived from the Latin verb agglutinare, which means "to glue together".
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 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 弟, from oto + hito (originally woto + pito, "young, younger" + "person"), and Japanese verbs, adjectives, the copula, and their affixes undergo sound transformations.