c. 1200, "a sentence, a brief passage of a written composition," from Old French clause "stipulation" (in a legal document), 12c., from Medieval Latin clausa "conclusion," used in the sense of classical Latin clausula "the end, a closing, termination," also "end of a sentence or a legal argument," from clausa, fem. noun from past participle of claudere "to close, to shut, to conclude" (see close (v.)).
Grammatical sense "one of the lesser sentences which united form a complex or compound sentence" is from c. 1300. Legal meaning "distinct condition, stipulation, or proviso" is recorded from late 14c. in English. The sense of "ending" mostly faded from the word between Latin and French, but it is occasionally found in Middle English.
A clause differs from a phrase in containing both a subject and its predicate, while a phrase is a group of two or more words not containing both these essential elements of a simple sentence. [Century Dictionary]