An inflection is when a word is modified for grammatical reasons (without changing categories, contrary to a derivation).

A typical inflection is carried out by adding grammatical affixes to a lexical or verbal root.

For instance, in French, when comparing “parlons” and “parlez“, we can easily identify the verbal root “parl-” and the suffixes “-ons” and “-ez“, respective markers of the first and second person plural.

Sometimes, the inflection of a word in a language doesn’t require the addition of a suffix or a prefix, but requires a sound change of the root such as “foot” (singular) and “feet” (plural) in English. In that case, we talk about internal inflection.