The Collaborative International Dictionary of English v.0.48:

Cuckoldry \Cuck"old*ry\ (-?ld-r?), n.
   The state of being a cuckold; the practice of making
   cuckolds.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cuckoldry
    n 1: the practice of making cuckolds; sexual conquests of
         married women

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "cuckoldry":
   adulterous affair, adultery, affair, amour, concubinage,
   criminal conversation, entanglement, eternal triangle,
   extracurricular sex, extramarital relations, flirtation,
   forbidden love, fornication, free love, free-lovism, hanky-panky,
   illicit love, incest, infidelity, intrigue, liaison, love affair,
   premarital relations, premarital sex, romance, romantic tie,
   triangle, unfaithfulness