A general term referring to the point in an object's orbit where it comes closest to its primary body (that is, the object around which it is orbiting). Orbital paths in general follow the form of an ellipse, wiith the primary body at one of the foci of that ellipse. The periapsis point is the point on this elliptical path that passes closest to the primary focus. The terms periapse or pericentre are sometimes used with the same meaning.
The opposite point on an orbit to a periapsis, the point at which the object is at its farthest possible distance from its primary, is termed the apoapsis (or the apoapse or apocentre). For objects with orbits that are relatively close to being circular in form (such as the Earth) there is little difference between these two values, but for more eccentric orbits (forming highly elongated ellipses) the variations can become extreme. Within the Solar System, eccentric orbits like this are common among periodic comets, whose periapsis will commonly lie close to the Sun, while their decades-long orbits can carry them out far out into the realm of the outer planets.
Periapsis is a general term, descriptive of the closest point of any object orbiting any other (or indeed of a comparable point on an abstract ellipse). For some specific bodies, there are specialised terms that are more commonly used. So, for example, for satellites orbiting Earth, their periapsis is usually referred to as their perigee. Similarly, for objects orbiting the Sun, this point is usually described as their perihelion.