Free Body Diagram: while a body or link is isolated, the forces (active and reactive) are also illustrated on it. It is called free body diagram of the body. The essential condition for this is that the body must be in equilibrium.
Friction Circle: This is a circle that drawn with hinge centre as the centre and radius equivalent to the product of radius of the pin and coefficient of friction.
Friction Axis: this is a line that is tangential to the friction circles and along which the compression or tension in the link acts.
Inertia Force: The magnitude of the inertia force is equivalent to the product of acceleration and mass & its direction is opposite to the direction of acceleration.