plane figure with five angles and five sides, 1560s, from Middle French pentagone or directly from Late Latin pentagonum "pentagon," from Greek pentagonon, noun use of neuter of adjective pentagonos "five-angled," from pente "five" (from PIE root *penkwe- "five") + gōnia "angle, corner" (from PIE root *genu- (1) "knee; angle"). The U.S. military headquarters Pentagon was completed 1942, so called for its shape; used allusively for "U.S. military leadership" from 1945. Related: Pentagonal.
In nature, pentagonal symmetry is rare in inanimate forms. Packed soap bubbles seem to strive for it but never quite succeed, and there are no mineral crystals with true pentagonal structures. But pentagonal geometry is basic to many living things, from roses and forget-me-nots to sea urchins and starfish. [Robert Bringhurst, "The Elements of Typographic Style," 1992]