A fusional language is a language in which one form of a morpheme can simultaneously encode several meanings.
Fusional languages may have a large number of morphemes in each word, but morpheme boundaries are difficult to identify because the morphemes are fused together.
Most European languages are somewhat fusional.
In Spanish the -ó in habló 'to speak' simultaneously codes indicative mode, third person, singular, past tense, and perfective aspect. If any one of these meaning components changes, the form of the verbal suffix must change.
The opposite of a highly fusional language is a highly agglutinative language.