Legally, adultery is generally defined as a married person's voluntary act of sexual intercourse with a person other than their spouse. Some older statutes limit adultery to sex with a married woman by a person other than her spouse, making it a form of human trespassing.
In most places, adultery is at least nominally a crime: in the US, it is generally classified as a misdemeanor, although prosecutions are not common since most district attorneys are too busy going after pot smokers.
The most important legal consequence of adultery, then, is that it is one of the grounds for seeking a divorce. For many years, it was the only ground for divorce in both the US and the UK (they took the Sermon on the Mount to heart). However, one could not obtain a divorce on an allegation of adultery: there had to be corroborating evidence. Which led to a number of clever little schemes for seeking a divorce, e.g.:
(A hotel room. Night.)
JOHN: Okay, Lucy, lower your dress under the sheets a bit so it will look like you're topless.
LUCY: How's that?
JOHN: That's good. Show a bit more cleavage, maybe. Yeah, perfect. Okay, I think that will do. HONEY!
(John's wife MARY opens the door and takes a photo of the two in bed.)
JOHN: (slips a fiver in Lucy's cleavage) Thanks for that. I couldn't take that damn marriage any more!
MARY: (slips another fiver in Lucy's cleavage) I can't see how the judge would doubt this one. Tell him I whacked John over the head, okay?
This isn't so common nowadays because most jurisdictions recognize alternate grounds for divorce (in many places, one can obtain a "no-fault divorce" for no stated reason at all). There are also certain defenses to adultery which vary from place to place (so contact a local lawyer before jumping in bed with someone). Some common ones are:
- Condonation - If one spouse continues the marital relationship normally (e.g. cohabitation) after learning of the other spouse's adultery, they cannot later raise adultery as grounds for divorce. This is one reason why, if your spouse is cheating on you, you might want to throw them out of the house and ask questions later.
- Recrimination - If both spouses are screwing around on each other, their adultery cannot be the ground for their divorce.
- Statute of limitations - Once a person becomes aware of their spouse's adultery, they only have a certain amount of time (usually a few years) to demand a divorce on those grounds.