Why are hinge joints called hinge joints?

Hinge joints are called so because they mimic a hinge, which is a mechanism on which something swings, limited to two directions, either back and forward or side to side. Think of a hinge on a door. It allows the door to open and close based on that one hinge. Door hinges do not allow the door to pivot or move in any direction besides open/close or front/back. Hinge joints do the same, such as the elbow and knee, which can only move forward and back.
Hinge joints are called that because they open and close like any hinge. If you look for one in the home and open and close it, you will see what is meant by a hinge joint. This joint allows movement in one plane only. The ankle and the knee are good examples. The knee is considered the largest one in the body.