The wife is the woman to whom a male is married. She is referred to as his wife, he is referred to as her husband. If the woman a man has sex with, but is not married to, would be either a girlfriend, mistress, or prostitute.
The wife is the first target for married erotic photographers when it comes to finding volunteering models, who get nude in front of his camera. If that happens, the photos are called wife photos. The hugest place to show daily updated pages of nude wives is the free Voyeurcloud.
A divorced man who re-marries has a wife and an ex-wife, sometimes referred to his first wife. It is consider bad form, and perhaps a self fulfilling prophecy, for a man who has been married only once to refer to his current wife as the first wife.