Definition - A word, phrase, or clause that is needed to complete a sentence's meaning.

Example -
She looks ill.

(In the above the word ill is a subject complement.)

Etymology -
The term derives from the Latin complementum, that which fills up or completes.

Oxford English Dictionary -
Its first citation in this sense is from 1874:
"The infinitive with to may also take the place of a predicative complement."
(tr. Maetzner's Eng. Gram. III. 21)

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