compound sentence
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)

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