Compulsion, n.
Etymology: [L. compulsio. See Compel.]
The act of compelling, or the state of being compelled; the act of driving or urging by force or by physical or moral constraint; subjection to force. If reasons were as plentiful as blackberries, I would give no man a reason upon compulsion. Shak. With what complusion and laborious flight We sunk thus low. Milton.
Syn. -- See Constraint.
Shades of meaning
Fernald has no entry of its own for compulsion; he lists it under necessity.
He lists it without comment; the entries above explain its neighbors.