To constrain means to hold back, limit, or restrict something or someone. When you're constrained by rules, those rules set boundaries on what you can do. When a budget constrains your choices, you can't spend as freely as you'd like.

Think of a dog on a leash: the leash constrains how far the dog can wander. In school, time constraints might force you to choose between finishing your science project and practicing piano. A small bedroom constrains how much furniture you can fit inside. Sometimes physical things constrain us, like when a cast constrains the movement of a broken arm. Other times, circumstances constrain our options, like when rainy weather constrains your outdoor plans.

The word often appears in the phrase constrained by, as in “The architect was constrained by the narrow lot” or “We're constrained by the amount of time we have.” A constraint is the thing doing the constraining: a rule, limitation, or restriction.

Being constrained isn't always bad. Sometimes constraints force us to be more creative. When you can only use certain materials for an art project, those constraints might lead you to solutions you wouldn't have discovered otherwise. Writers often say that constraints like rhyme schemes or word limits make their work sharper and more focused.