Use the adjective posterior to describe something that's in the back. It’s often used in anatomy — a posterior cerebral artery supplies blood to the back of the brain, and the tail is on the posterior of a fish.
The prefix post means “after,” and things that are posterior come after the things in the front. The opposite of posterior is anterior, which refers to the front, usually of body parts. You can also use the word posterior to jokingly refer to the part of your body that you sit on — your backside. Posterior is also a formal way to say later in time.