|Definition - A sentence that consists of two or more independent clauses.|
Example - The problem was interesting, but I could never find the answer.
In the above, both
(1) The problem was interesting and
(2) I could never find the answer are independent clauses — i.e. they can each stand on their own as sentences.
Oxford English Dictionary - The term's first citation is from 1772:
"…. We now proceed to compound sentences, wherein to avoid inaccuracy is required the greatest judgement and attention.…"
(A. Bayly Gram. Eng. Lang. 80)