Definition
Derivation is the formation of a new word or inflectable stem from another word or stem. It typically occurs by the addition of an affix.
The derived word is often of a different word class from the original. It may thus take the inflectional affixes of the new word class.
Discussion
In contrast to inflection, derivation
• is not obligatory
• typically produces a greater change of meaning from the original form, and
• is more likely to result in a form which has a somewhat idiosyncratic meaning.
• often changes the grammatical category of a root