The term "hinge line" refers to the imaginary line that runs through the middle of a hinged object, determining the point around which it rotates. This line is also known as the pivot axis or axis of rotation, and the term can be used interchangeably with "hinge line." Other synonyms for this term include "revolute joint," "fulcrum," and "pivot point." These terms are commonly used in fields such as mechanical engineering, physics, and robotics to describe the motion of hinge-like objects, such as doors, levers, and robotic arms. Understanding the concept of the hinge line is essential for designing and constructing effective hinged mechanisms that are both stable and functional.