The path traced by the instantaneous center of a plane figure when it undergoes plane motion. If a plane rigid body is constrained to move in its own plane but is otherwise free to undergo an arbitrary translational and rotational motion, it is found that at any instant there exists a point, called the instantaneous center, about which the body is rotating. The path that this instantaneous center traces out in space as the motion unfolds is called the space centrode. The path that it would trace out in a coordinate system which is rigidly attached to the body is called the body centrode. The motion may therefore be specified, when the two centrodes are given, by allowing one curve to roll without slipping along the other.