device designed to transform input forces and movement into a desired set of output forces and movement

A mechanism is a mechanical device for doing something. It includes the idea of tools and machines, but is used for a wider range of objects, processes and ideas.

Palaeolithic hand axes were one of the first tools used. Simple modern mechanisms would be rope pulleys or levers. The internal combustion engine is a mechanism for turning oil into motion. A complicated example would be computers and the web.

The word "mechanism" is also used as a metaphor. The celestial mechanics of Kepler and Newton is a way of predicting and explaining planetary motions in the Solar System. Darwin's natural selection is his proposed mechanism as to how evolution took place.

In that way, a mechanism in science or technology is an explanation of how something works or might work.