Apoapsis is the point in an orbit around a celestial body where the orbiting object is farthest from the center of the body. It is a key concept in understanding elliptical orbits.
Periapsis: The point in an orbit closest to the center of mass of the celestial body being orbited.
Eccentricity: A measure of how much an orbit deviates from being circular; calculated as $e = \frac{c}{a}$ where $c$ is the distance from the center to a focus and $a$ is the semi-major axis.
Semi-Major Axis: The longest radius of an ellipse, running from its center through one of its foci to its perimeter.