Y-Axis

the "vertical" axis when constructing and plotting a graph

Taken from Dictionary of Science

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The vertical axis of a two-dimensional plot in Cartesian coordinates. Physicists and astronomers sometimes call this axis the ordinate, although that term is more commonly used to refer to coordinates along the -axis.

Taken from Wolfram MathWorld

X-Axis

the "horizontal" axis when constructing and plotting a graph.

Taken from Dictionary of Science

The coordinate surfaces of the Cartesian coordinates (x, y, z). The z-axis is vertical and the x-axis is highlighted in green. Thus, the red plane shows the points with x=1, the blue plane shows the points with z=1, and the yellow plane shows the points with y=-1. The three surfaces intersect at the point P (shown as a black sphere) with the Cartesian coordinates (1, -1, 1).

Taken from Wikipedia