[pen-tuh-gon, -guh n] /ˈpɛn təˌgɒn, -gən/

a polygon having five angles and five sides.
the Pentagon.

a polygon having five sides
the five-sided building in Arlington, Virginia, that houses the headquarters of the US Department of Defense
the military leadership of the US

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” (see five) + gonia “angle” (see knee (n.)). 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]

A polygon having five sides.

pentagonal adjective (pěn-tāg’ə-nəl)

An immense five-sided building in Virginia, just outside Washington, D.C., that serves as headquarters for the Department of Defense.

Note: The term is often used to refer to the Department of Defense or the military: “The Pentagon agreed today to submit the modified weapons plan to the president.”

Note: The Pentagon was severely damaged by the September 11 attacks.

A polygon having five sides.

Note: The Pentagon is a huge five-sided building near Washington, D.C., that contains offices of the Department of Defense.


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  • Pentagon papers

    A classified study of the Vietnam War that was carried out by the Department of Defense. An official of the department, Daniel Ellsberg, gave copies of the study in 1971 to the New York Times and Washington Post. The Supreme Court upheld the right of the newspapers to publish the documents. In response, President Richard […]

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