Voyeurism is the sexual interest in or practice of spying on people engaged in intimate behaviors, such as undressing, sexual activity, or other actions usually considered to be of a private nature.

The principal characteristic of voyeurism is that the voyeur does not normally relate directly with the subject of his/her interest, who is often unaware of being observed. Voyeurism may involve the making of a secret photograph or video of the subject during an intimate activity. When the interest in a particular subject is obsessive, the behavior may be described as stalking.

The term comes from the French voyeur, "one who looks". A male voyeur is commonly labeled "Peeping Tom", a term which originates from the Lady Godiva legend. However, that term is usually applied to a male who observes somebody through her window, and not in a public place.

(source: WiKiPedia)