Definition of "radian" [ra•di•an]

• A unit of angular measure equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle, approximately 57°17ʹ44.6〞. See Table at measurement. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Light en - ergy gathered by the sheaths was relayed by lines called radian draws."
• "A radian is a measurement used to describe angles."
• "A radian is the equivalent of (a) 2 angular degrees (b) 1/(2) of the angle comprising a full circle (c) the circumference of a unit circle (d) 1 = 4 of the angle comprising a full circle"