C-spanner

noun

a sickle-shaped spanner having a projection at the end of the curve, used for turning large narrow nuts that have an indentation into which the projection on the spanner fits

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012