monogamy

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

• noun The practice or condition of having a single sexual partner during a period of time.
• noun The practice or condition of being married to only one person at a time.
• noun The practice of marrying only once in a lifetime.
• noun Zoology The condition of having only one mate during a breeding season or during the breeding life of a pair.

from The Century Dictionary.

• noun The practice of marrying only once, or the principle which upholds that practice; the principle that forbids remarriage after the death of a former husband or wife: opposed to digamy. See bigamy, 2.
• noun The condition of being married to only one person at one time: opposed to bigamy or polygamy. See bigamy, 1.
• noun In zoology, the habit of having only one mate; the habit of living in pairs; the paired state.

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

• noun Single marriage; marriage with but one person, husband or wife, at the same time; -- opposed to polygamy. Also, one marriage only during life; -- opposed to deuterogamy.
• noun (Zoöl.) State of being paired with a single mate.

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

• noun A form of sexual bonding involving a permanent pair bond between two beings.

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

• noun having only one spouse at a time