It can be just about anything, depending what you are graphing.
y axis
It is the y-axis and the horizontal line is the x-axis
The x and y axis cross at (0,0) on the coordinate plane.
y-axis
It is the x axis
A coordinate plane consists of the y axis up and left and the x axis right and down
The horizontal number line on a coordinate plane is called the x-axis, and the vertical number line on a coordinate plane is called the y-axis.
Coordinate Plane
The vertical axis in a plane coordinate graph
I believe you mean the x-axis, the horizontal axis on a coordinate plane
X has two lines that cross. Think of a-cross. X axis is horizontal on a coordinate plane. Y has a vertical line... the Y axis is the vertical on a coordinate plane.
The y axis