Density refers to how closely packed or crowded something is within a given area or volume. In calculus, it can refer to how closely spaced the slope field lines are in relation to each other.
Related terms
Rate of Change: How quickly one quantity changes in relation to another over time.
Gradient Vector Field: A vector field that represents both magnitude and direction at each point on its domain.
Level Curve/Contour Line: A curve on which all points have equal values for some given function.