criminal conversation

A tort under common law that involved the seduction of another person's spouse.

A few states still permit a lawsuit for damages by the injured spouse against the wrongdoer. Many states have abolished this action.

Criminal conversation is not the same as alienation of affection, which does not necessarily involve the commission of adultery.

The noun has one meaning:

Meaning #1: extramarital sex that willfully and maliciously interferes with marriage relations
  Synonyms: adultery, fornication