Entry 1 sense Webster, 1913 Radian /(rā"dĭ*an)/ · Ra·di·an · IPA /ˈɹeɪdiən/ 01 n. An arc of a circle which is equal to the radius, or the angle measured by such an arc. 1. An arc of a circle which is equal to the radius, or the angle measured by such an arc.(Math.)