Derivational Affix

A derivational affix is an affix that changes the meaning or word class of a root word.

Example: Change in Meaning
Adding the prefix in- to the root word visible to make invisible changes the meaning to the opposite of visible.

Example: Change in Word Class
Adding the suffix -ity to the root word visible (Adjective) to make visibility (Noun) changes the word class of the word from an adjective to a noun.

Order
Derivational affixes are usually added to a root before Inflectional affixes.