Mechanism, n.
Etymology: [Cf. F. mécanisme, L. mechanisma. See Mechanic.]
1. The arrangement or relation of the parts of a machine; the parts of a machine, taken collectively; the arrangement or relation of the parts of anything as adapted to produce an effect; as, the mechanism of a watch; the mechanism of a sewing machine; the mechanism of a seed pod.
2. Mechanical operation or action. He acknowledges nothing besides matter and motion; so that all must be performed either by mechanism or accident. Bentley.
3. (Kinematics)
An ideal machine; a combination of movable bodies constituting a machine, but considered only with regard to relative movements.
Shades of meaning
Fernald has no entry of its own for mechanism; he lists it under tool.
Where he writes about it:
Mechanism is a word of wide meaning, denoting any combination of mechanical devices for united action. — from tool
A chisel by itself is a tool; when it is set so as to be operated by a crank and pitman, the entire mechanism is called a machine; as, a mortising-machine. — from tool