Definition - The modifying of a word to show grammatical information such as possession, person, mood, tense, or number.
Note: This is in contrast to derivation where affixes are used to create new words.

Example -
I want --> I wanted
(In the above the inflection changes the tense.)

Etymology -
The word derives from the Latin inflectere, to bend in or change.

Oxford English Dictionary -
Its first citation in its grammatical sense is from 1668:
"The rules which are proper and peculiar to any one Language … about the Inflexion of words, and the Government of cases."
(Wilkins Real Char., 297)

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