Definitions for y-axisˈwaɪˌæk sɪs; ˈwaɪˌæk siz
Random House Webster's College Dictionary
y-ax•isˈwaɪˌæk sɪs; ˈwaɪˌæk siz(n.)(pl.)y-ax•es
(in a plane Cartesian coordinate system) the axis, usu. vertical, along which the ordinate is measured and from which the abscissa is measured.
(in a three-dimensional Cartesian coordinate system) the axis along which values of y are measured and at which both x and z equal zero.
Origin of y-axis:
the vertical axis in a plane coordinate system
The axis on a graph that is usually drawn from bottom to top and usually shows the range of values of variable dependent on one other variable, or the second of two independent variables.