definition

From Middle English diffinicioun, borrowed from Middle French definition, which came from Latin dēfīnītiō, itself from the verb dēfīniō. In English it lines up neatly as define plus the suffix -ition.

The word passed from Latin into Middle French and then into Middle English as diffinicioun, settling over time into the modern spelling definition.

It's built from the same root as the verb define, with -ition marking it as the noun form of that action.