obligation

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

• noun A social, legal, or moral requirement, such as a duty, contract, or promise, that compels one to follow or avoid a particular course of action.
• noun The constraining power of a promise, contract, law, or sense of duty.
• noun Law A document in which a person binds himself or herself to undertake or refrain from doing a particular act.
• noun A debt instrument, such as a loan, mortgage, or bond.
• noun The state, fact, or feeling of being indebted to another for a special service or favor received.

from The Century Dictionary.

• noun The constraining power or authoritative character of a duty, a moral precept, a civil law, or a promise or contract voluntarily made; action upon the will by a sense of moral constraint.
• noun That to which one is bound; that which one is bound or obliged to do, especially by moral or legal claims; a duty.
• noun A claim; a ground of demanding.
• noun The state or fact of being bound or morally constrained by gratitude to requite benefits; moral indebtedness.
• noun In law: A bond containing a penalty, with a condition annexed, for payment of money, performance of covenants, or the like: sometimes styled a writing obligatory. By some modern English jurists the word is used as equivalent to legal duty generally.
• noun In Roman law, the juridical relation between two or more persons in virtue of which one can compel the other to do or not to do a certain act which has a monetary value, or can at least be measured by a monetary standard.
• noun In medieval schools, a rule of disputation by which the opponent was bound to admit any premise, not involving a contradiction, begging of the question, or other fallacy, which the respondent might propose.
• noun Synonyms Engagement, contract, agreement.

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

• noun The act of obligating.
• noun That which obligates or constrains; the binding power of a promise, contract, oath, or vow, or of law; that which constitutes legal or moral duty.
• noun Any act by which a person becomes bound to do something to or for another, or to forbear something; external duties imposed by law, promise, or contract, by the relations of society, or by courtesy, kindness, etc.
• noun The state of being obligated or bound; the state of being indebted for an act of favor or kindness; -- often used with under to indicate being in that state.
• noun (Law) A bond with a condition annexed, and a penalty for nonfulfillment. In a larger sense, it is an acknowledgment of a duty to pay a certain sum or do a certain things.
• noun See under Day.
• noun in a state of obligation{4}.

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

• noun The act of binding oneself by a social, legal, or moral tie to someone.
• noun A social, legal, or moral requirement, duty, contract, or promise that compels someone to follow or avoid a particular course of action.
• noun A course of action imposed by society, law, or conscience by which someone is bound or restricted.
• noun law A legal agreement stipulating a specified payment or action; the document containing such agreement.

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

• noun a personal relation in which one is indebted for a service or favor
• noun the social force that binds you to the courses of action demanded by that force
• noun a written promise to repay a debt
• noun a legal agreement specifying a payment or action and the penalty for failure to comply
• noun the state of being obligated to do or pay something