Apocentre distance ($Q$)

The apocentre distance is the greatest distance from the attracting centre on a bound Kepler orbit. For an elliptic orbit,where $q$ is pericentre distance. Thus the two turning distances determine both semi-major axis and orbital eccentricity.

A particle moves more slowly near apoapsis and spends more time there, an important effect when translating an orbital population into observed spatial counts. A parabolic or hyperbolic escape trajectory has no finite outer turning point, so assigning it an elliptic apocentre distance would misdescribe the motion.