/ ˈpentəgən; NAmE -gɑːn/ noun
1.
[ C ] ( geometry ) a flat shape with five straight sides and five angles
2.
the Pentagon [ sing. ] the building near Washington DC that is the headquarters of the US Department of Defence and the military leaders :
a spokesman for the Pentagon
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WORD ORIGIN
late 16th cent.: via Latin from Greek pentagōnon , neuter (used as a noun) of pentagōnos five-angled.