D defs.my
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. 1.
    An arc of a circle which is equal to the radius, or the angle measured by such an arc.(Math.)