The closed path energy travels — a loop that works because it returns; in practice, one full round of a course.
Latin circuitus, from circumire (to go around), circum (around) + ire (to go).
The return leg is not incidental to the loop; it is the part that lets everything upstream of it work at all — a path that never closes is a dead end rather than a circuit, and a break anywhere on one stops the whole course, not just the piece after the break.