On a graph, the y-axis is the line that runs up and down the side, with numbers marked along it. Its partner is the x-axis, the line that runs left to right along the bottom. Together they let you fix any point on the graph: how far across (x) and how far up (y). Usually the y-axis carries the thing you measured — sales, temperature, height — while the x-axis carries what you measured it against, most often time. So in "sales rose sharply", the rise you see is movement up the y-axis. It is a countable noun, and it almost always takes the: the y-axis, plural the y-axes (say "AX-eez"). Common phrases: on the y-axis, along the y-axis, label the y-axis, the y-axis shows temperature in degrees. Written with a small y and a hyphen; you may also see the y axis with no hyphen, and in books the letter is often italic. In maths, the y-axis is also the vertical line through the middle of a coordinate grid — the line where x = 0 — with positive numbers above zero and negative numbers below.