A vertical line on a graph. It can be located to the right or left, or even in the middle. Scientists label the y-axis with whatever they are measuring. Perhaps it is time, or a population, or grams of a substance. The y-axis and its horizontal partner, the x-axis, tell us what data presented in the graph represents.
In a sentence
When trying to float a candy bar, my graph of the results showed the mass of the candy on the y-axis.
axis The line about which something rotates. On a wheel, the axis would go straight through the center and stick out on either side. (in mathematics) An axis is a line to the side or bottom of a graph; it is labeled to explain the graph’s meaning and the units of measurement.
x-axis (in mathematics) The horizontal line at the bottom of a graph, which can be labeled to give information about what the graph represents.
y-axis (in mathematics) The vertical line to the left or right of a graph, which can be labeled to give information about what the graph represents.