Definition - A letter or letters put at the beginning, end, middle, etc. of a word or a stem to create a new word.

Example -

(1) The -ly in quickly is a type of affix called a suffix.
(2) The re- in recharge is a type of affix called a prefix.

Etymology -
The term derives from the Latin affixus, fastened to (from ad, to + figere, fasten).

Oxford English Dictionary -
The first citation for the noun is from 1612:
"Framing it somewhat to their own country fashion, in notation of points, affixes, conjugations."
( Brerewood Lang. & Relig. ix. 76)

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