Held within limits. Constraint narrows what is possible and is often what makes a design decidable.
Latin constringere (to bind tightly), from com- (together) + stringere (to draw tight, bind); held within limits set from outside, which narrows what remains possible.
Latin stringere is to draw tight, and something constrained is literally bound, not merely limited by circumstance. A design pulled tight enough to bind loses options, but often gains the one thing an open design cannot have: a decidable answer.