y-axis

The y-axis is the vertical “up-and-down line” of the coordinate plane. Walking along it, you move higher or lower but never left or right.

Every point on the y-axis has x-coordinate 0, because staying on this line means you never stepped sideways. Its points look like (0, y): for example (0, 3), (0, -2), and the origin (0, 0). Numbers grow positive going up and negative going down.

Where a graph crosses the y-axis is its y-intercept. For a function, there is at most one such crossing, since a single input x = 0 can produce only one output.