constraint

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The threat or use of force to prevent, restrict, or dictate the action or thought of others.
• noun The state of being restricted or confined within prescribed bounds.
• noun One that restricts, limits, or regulates; a check.
• noun Embarrassed reserve or reticence; awkwardness.

from The Century Dictionary.

• noun Irresistible force, or its effect; any force or power, physical or moral, which compels to act or to forbear action; compulsion; coercion; restraint.
• noun Specifically Repression of emotion, or of the expression of one's thoughts and feelings; hence, embarrassment: as, he spoke with constraint.
• noun In analytical mechanics, the product of the mass of a particle into the square of that velocity which, compounded with the velocity the particle would have if free, would give the actual velocity.
• noun Synonyms Violence, necessity, coercion. See force, n.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of constraining, or the state of being constrained; that which compels to, or restrains from, action; compulsion; restraint; necessity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Something that constrains.
• noun mathematics A condition that a solution to an optimization problem must satisfy.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the state of being physically constrained
• noun a device that retards something's motion
• noun the act of constraining; the threat or use of force to control the thoughts or behavior of others