Chiefly with of. The source or origin of a particular quality, action, state of affairs, etc.

The place at which a spring emerges from the ground; the headspring or source of a stream or river.

Figurative and in figurative contexts: a thing likened to the source of a stream or river.

A surface, projection, etc., that is part of or attached to a spring, by means of which the spring acts upon another component of a mechanism.