|Definition - The area of grammar that deals with inflection — i.e. the modifying of a word to show grammatical information, such as possession, tense, or number.|
Etymology - The word was coined by misspelling the word accidents, which meant inflections. The latter derives from the Latin accidentia, a grammatical term used by Quintilian.
Oxford English Dictionary - Its first citation is from 1509:
"Dame Gramer … taught me ryght well Fyrst my Donet and then my accidence."
(Hawes Past. Pleas. (1845) v. ix. 23)