Usually the solenoid. Could be the starter. If one were to energize the starter and it spins right away, see #1. If not, service battery cables and chassis grounds. Model info could be helpful if you need more assistance.

Good suggestions, I would do the cheap things first. Jumping directly from a known good battery to the starter, even using one side at a time them both, may help narrow down where the trouble is or isn't. Ground cable to frame connection is often overlooked and the problem.

Walt Conner