Apsides

In a heliocentric orbit, the point furthest from the Sun (S) is called aphelion (A), and the point nearest to the Sun is called perihelion (P), as shown in the figure. These two points define the line of apsides [singular: apse] which passes through the Sun, and is an axis of symmetry of the path. When the path is discovered to be an ellipse this line is identified as its major axis.

The corresponding points that arose originally in geocentric astronomy are (still) called 'apogee' and 'perigee'. Kepler invented the names 'aphelion' and 'perihelion' by analogy when he needed them for his heliocentric astronomy (from the Greek: helios = Sun, ge- referring to Earth, with prefixes ap(o)- = away from, peri- = around / near).