|Definition - A noun, pronoun, or adjective that modifies a direct object.|
Example - I consider the lecturer stupid.
(The word lecturer is the direct object of the verb consider; stupid modifies this direct object, hence it is an object complement)
Oxford English Dictionary - The term's first citation is from 1906:
"An object complement is a noun or adjective completing the meaning of a transitive verb."
(G. R. Carpenter Eng. Gram. ii. 25)