Eccentricity is the distance between the foci divided by the length of the major axis and is a number between zero and one. An eccentricity of zero indicates a circle. An inclination of 90 degrees indicates a polar orbit. An inclination of 180 degrees indicates a retrograde equatorial orbit. A retrograde orbit is one in which a satellite moves in a direction opposite to the rotation of its primary.
Periapsis is the point in an orbit closest to the primary.
The opposite of periapsis, the farthest point in an orbit, is called apoapsis . Periapsis and apoapsis are usually modified to apply to the body being orbited, such as perihelion and aphelion for the Sun, perigee and apogee for Earth, perijove and apojove for Jupiter, perilune and apolune for the Moon, etc. The time of periapsis passage is the time in which a satellite moves through its point of periapsis. Nodes are the points where an orbit crosses a plane, such as a satellite crossing the Earth's equatorial plane. The longitude of the ascending node is the node's celestial longitude.