On a graph, the line that runs from left to right, along the bottom, with numbers or dates or names marked along it. Say it "EX-axis." It is one half of a pair: the x-axis goes across, the y-axis goes up. Whatever you are measuring against — time, age, month, temperature — usually sits on the x-axis, and the thing that changes because of it sits on the y-axis. So on a chart of a company's sales, the months run along the x-axis and the money climbs the y-axis. A useful memory trick many teachers here use: the x is a cross, and you go across. The plural is x-axes, said "EX-ax-eez," which sounds nothing like the singular and surprises people. You will meet the word all through school mathematics and science, and also in ordinary work life whenever somebody is explaining a chart in a meeting or a report. It is written with a small x and a hyphen; some books write it as the x axis with a space, and both are fine.