A point. In fact it is fixed point and the locus of all points, in a plane that is a fixed distance from that fixed point defines the circle.

The fulcrum is the fixed part of a lever. (the pivot point)

upper fixed point is a temperature of stem from water boiling and standards atmospheric pressure

lower fixed point is the temperature of pure melting ice.

A lever is a stiff structure that rotates around a fixed point. The fixed point around which a lever rotates is fulcrum.

The fixed point that is located in the center of a circle and is used as the guiding point to draw it is called the "point of origin".