An ELT Glossary : Subject Complement / Object Complement
A complement is a clause element which in some way equates with or relates to either the subject or object of the clause. Complements may be adjectives or noun phrases.
a) Subject complements occur after copulative verbs :
Mary is a neurologist ( Mary = a neurologist).
John looked unhappy (John = unhappy)
The leaves are turning brown ( the leaves = brown)
b) Object complements
I can prove you wrong ( you = wrong)
She called him a fool (him = a fool)