Knowing that "selene" refers to our moon (as in "selenography"), you can probably figure out what these words mean. These terms are most commonly used in matters related to spaceflight. The periselene point in an elliptical orbit is the point of closest approach to the moon, while aposelene is the farthest point in the orbit. When a spacecraft arrives at the moon, it usually assumes an elliptical capture orbit. If the mission involves mapping, as was the case with the Lunar Prospector and Clementine spacecraft, then a later maneuver circularizes the orbit.