Eccentric, in Geometry, denotes two circles or spheres, one of which is contained within the other, but the centres of the two do not coincide. In mechanics, the name is given to a contrivance frequently substituted for a crank, for obtaining a reciprocating motion from a circular. It consists of a circular disc placed eccentrically upon a shaft, and revolving within a hoop formed at one end of the connecting rod. The valves of most steam engines which work with a fly-wheel are moved by an eccentric.