cuckold

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

• noun A man whose wife is unfaithful.
• transitive verb To make a cuckold of.

from The Century Dictionary.

• To dishonor by adultery: said of a wife or her paramour.
• noun A corrupt form of cockle.
• noun A man whose wife is false to him; the husband of an adulteress.
• noun A book-name of the cow-bird, Molothrus ater; so called from its parasitic and polygamous habits.
• noun A name of the cow-fish, Ostracion quadricore: apparently so called from its horns. See cow-fish .

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

• noun A man whose wife is unfaithful; the husband of an adulteress.
• noun A West Indian plectognath fish (Ostracion triqueter).
• noun The cowfish.
• transitive verb To make a cuckold of, as a husband, by seducing his wife, or by her becoming an adulteress.

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

• noun A man married to an unfaithful wife, especially when he is unaware or unaccepting of the fact.
• verb transitive To make a cuckold of someone by being unfaithful, or by seducing his wife.

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

• verb be sexually unfaithful to one's partner in marriage
• noun a man whose wife committed adultery