Definition:Apsis

Definition
Let $A$ be a body in orbit $O$ around another body $B$.

Let $\mathbf r$ be the radius of $O$.

An apsis is a point on $O$ at which:
 * the rate of change of the length of $\mathbf r$ is zero

and:
 * the velocity of $A$ is perpendicular to $\mathbf r$.