Chapter 1 - Graphs - 1.2 Graphs of Equations in Two Variables; Intercepts; Symmetry - 1.2 Assess Your Understanding - Page 16: 5

y-axis

If the point $(x,y)$ is on a graph that is symmetric with respect to the y axis, the point $(-x,y)$ is also on that graph. Thus, if the point $(-x, y)$ is also on the graph of an equation for every point $(x, y)$, then the graph is symmetric with respect to the y-axis.