In two-dimensional geometry, the line represented by the equation y=0.

Actually, it's not necessary that the equation be y=0, but rather y=n, with n a constant. Further, in a polar graph, the x-axis is often the outer circle, and therefore the equation is r=n, where r is the radius and n a constant.

Some folks use the terms ordinate and abcissa to refer to the axes. Usually these folks are high school math teachers who have run out of interesting things to teach.

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