perilune
pericynthion

The perilune is in front of the Moon; the spacecraft moves there in the same direction as the Moon.

At perilune at 05:12 UT on 20 September, the main braking engine was fired, initiating the descent to the lunar surface.

At the end of the twelfth orbit, Worden performed a circularization burn with SPS, raising the perilune of his orbit.

After two orbital adjustments were performed on 18 September and 19 September the perilune was decreased to 15.1km, as well as the inclination altered in preparation for landing.

The spacecraft made many orbit adjustments over its 18 month lifetime in order to optimize the operation of various experiments, lowering the perilune to as little as 25 km.

Another burn placed it in a polar orbit around the Moon, with burns at the perilune of the first three orbits decreasing the apolune until it entered a final circular orbit.

Mission Control predicted that by the time the crew awoke the next "morning", the lowest point of their orbit, perilune, would have dropped to about 8.6 nm (16.1 km) but this turned out to be an underestimate.

The altitude at apoapsis (point farthest from the surface) for a lunar orbit is known as apolune, apocynthion or aposelene, while the periapsis (point closest to the surface) is known as perilune, pericynthion or periselene.

Apollo 13 was the only Apollo mission to actually turn around the Moon in a free-return trajectory (however, two hours after perilune, propulsion was applied to speed the return to Earth by 10 hours and move the landing spot from the Indian Ocean to the Pacific Ocean).

There are also specific terms for orbits around particular bodies; things orbiting the Sun have a perihelion and aphelion, things orbiting the Earth have a perigee and apogee, and things orbiting the Moon have a perilune and apolune (or periselene and aposelene respectively).