On the coordinate plane, the x-axis is the second number of an ordered pair

The correct answer is: y-coordinate.

Explanation: In an ordered pair (x, y), the first number represents the x-coordinate, which corresponds to the horizontal position on the graph. The second number, or y-coordinate, represents the vertical position. The x-axis is the line where y = 0, and the y-axis is the line where x = 0.