When a body is orbiting around another body, the perihelion is the moment in that orbit when it becomes closest to its subject. The Earth is at its perihelion to the Sun in early January.
The opposite of perihelion is “aphelion,” which is when the body is the furthest from its subject.
Why I Looked It Up
In an episode of Buck Rogers, bad things were going to happen when The Searcher reached its perihelion with some star.