What does non linear graph mean?

a non linear graph pretty much means the points on the graph do not create a straight line. in order for a graph to be linear both the x axis and y axis have to increase/decrease at a constant rate, while in a non-linear graph when you connect the point it creates a line that isn't straight. however, in experimental data if the points are a bit off, but still in one general area, it can be considered linear.