|Definition - A word, phrase, or clause that both follows a copula and modifies the subject of the sentence.|
The moon was a ghostly galleon.
(In the above, was is a copula which links the subject complement ghostly galleon to the subject moon.)
Oxford English Dictionary - The term's first citation is from 1939:
"A certain number of adverbs may be used as *subject-complements, i.e. as complements to … verbs of incomplete predication."
(H. E. Palmer Gram. Spoken Eng. (ed. 2) ii. 80)