the fixed line, usually horizontal, from which the angle made by the radius vector is measured in a polar coordinate system.
Origin of polar axis
First recorded in 1890–95
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for polar axis
the fixed line in a system of polar coordinates from which the polar angle, θ, is measured anticlockwise
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