noun
straight angle
A mathematical term for an angle of 180 degrees, literally 'two right angles'. Used in geometry contexts.
二直角は180度に等しい。
A straight angle equals 180 degrees.
平角 is a more common term for 'straight angle' in modern Japanese geometry; 二直角 is less frequent and more literal.
Compound of 二 (ni, 'two') + 直角 (chokkaku, 'right angle'), literally 'two right angles'. The term is a direct calque from the mathematical concept of a straight angle.