Infidelity can be defined as any violation of the mutually agreed-upon rules or boundaries of a relationship, and is a breach of faith in an inter-personal relationship.

Sexual infidelity in marriage is sometimes called adultery, philandery or an affair and in other inter-personal relationships it is sometimes called cheating. A man whose wife has committed adultery is referred to as a cuckold, but no equivalent word exists for a woman whose partner has cheated.

Infidelity is not inherently sexual nature, although certain acts of infidelity could be sexual.

What constitutes an act of infidelity varies between and within cultures and depends also on the type of relationship that exists between people. Even within an open relationship, infidelity may arise if a partner to the relationship acts outside of the understood boundaries of the relationship. (See Blumstein.)

Incidence of infidelity

Some authorities (for example Frank Pittman in 'Grow Up' Golden Books) observe infidelity is involved in 90% of first time divorces. A 1997 study with Kristina Gordon found 'more than half of the marriages that experience infidelity ended in divorce'. By contrast John Gottman with his 35 years of research into marriage, is reported as saying "Only 20 percent of divorces are caused by an affair. Most marriages die with a whimper, as people turn away from one another, slowly growing apart." Fifty United Kingdom divorce lawyers were asked to name the most common causes of their cases in 2003. Of those who cited extramarital affairs, 55% said it was usually the husbands and 45% said that it was the wives who cheated.

In addition between 10-15% of children are conceived as a result of an affair.

Infidelity that does not involve sex or conception may be referred to as a romantic friendship or an emotional affair. Some people consider virtual sex, which is an impersonal on-line relationship, as infidelity.

Infidelity at work

An office romance, work romance, or corporate affair is a romance that occurs between two people who work together in the same office, work location, or business.

Though office romances are generally considered to be unhelpful to business and work relationships, inter-personal relationships at work are common. These relationships are usually discouraged as part of company policy, and at times even prohibited.

The suspicion that an advantage is gained by 'sleeping with the boss' in a competitive environment ensures that these relationships occur by stealth.

A selection of infidelity in film and theatre

Infidelity has been a theme in many films and in the theatre, including -

• The Apostle
• American Beauty
• Book of Love
• Brief Encounter
• Brokeback Mountain
• Closer
• The Constant Gardener
• The Crucified Lovers
• Dr Zhivago
• Design for Living
• Derailed
• Diary of a Mad Black Woman
• Double Jeopardy
• Emotional Arithmetic
• Eyes Wide Shut
• Fracture
• The Bridges of Madison County
• The English Patient
• The Flower of My Secret (La Flor de Mi Secreto)
• Fatal Attraction
• Goodfellas
• Hamlet
• Magnolia
• Much Ado About Nothing
• They All Laughed
• Lake Placid
• Rain
• Random Hearts
• St. Elmo's Fire
• Revenge
• Disclosure
• She-Devil
• The Perfect Murder
• The Postman Always Rings Twice
• Ruthless People
• Thelma & Louise
• Unfaithful (film)
• Waiting to Exhale
• Match Point
• A Walk on the Moon