Figure 1. Perihelion.
Figure 2. Periapsis and apoapsis.
The perihelion is the point in the solar orbit of a planet, comet, or other object when it is nearest to the Sun (see Figure 1). The perihelion distance is the distance between the object and the Sun at this point. The time of perihelion passage is the time at which an object is at perihelion.
Argument of perihelion
The argument of perihelion is the angle between the ascending node and the perihelion of an orbit around the Sun, measured in the orbital plane and in the direction of orbital motion. It specifies the direction of the major axis of an orbit around the Sun and is one of the orbital elements.
The aphelion is the point in the orbit of a planet, or other object in the Solar System, which is furthest from the Sun.
Apsis, periapsis, and apoapsis
An apsis is either of the points in the orbit of a celestial object that is closest to (periapsis) or farthest from (apoapsis) the body being orbited (see Figure 2). Special names are given to these points for commonly referenced systems, including periastron and apoastron (in the case of stars), perigee and apogee (in the case of Earth), and, as in the case of the Sun, as described above, perihelion and aphelion.