steradianIf A = r2 then solid angle ? = 1 sr.ste·ra·di·an (st?-r??d?-?n)n. Abbr. sr A unit of measure equal to the solid angle subtended at the center of a sphere by an area on the surface of the sphere that is equal to the radius squared. The total solid angle of a sphere is 4? steradians.[ste(reo)- + radian.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.steradian (st??re?d??n) n (Units) an SI unit of solid angle; the angle that, having its vertex in the centre of a sphere, cuts off an area of the surface of the sphere equal to the square of the length of the radius. Symbol: sr [C19: from stereo- + radian]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014