In clinical psychology, voyeurism is the sexual interest in or practice of spying on people engaged in intimate behaviors, such as undressing, sexual activity, or other activity usually considered to be of a private nature. In popular imagination the term is used in a more general sense to refer to someone who habitually observes others without their knowledge, with no necessary implication of sexual interest.
Voyeurism (from the French voyeur, "one who looks") can take several forms, but its principal characteristic is that the voyeur does not normally relate directly with the subject of their interest, who is often unaware of being observed. The practice of making a permanent image of an intimate activity has been made easier with modern photographic and video technology and up skirt and down blouse photography is considered an invasion of privacy. However, in today's society the concept of voyeurism has evolved, especially in popular culture. Non-pornographic 'reality' television programs such as Survivor and The Real World, are prime examples of voyeurism, where viewers (the voyeur) are granted an intimate interaction with a subject group or individual. Although not necessarily "voyeurism" in its original definition, as individuals in these given situations are aware of their audience, the concept behind "reality TV" is to allow unscripted social interaction with limited outside interference or influence. As such, the term still maintains its sexual connotations and is, as such, outdated.
Male voyeurs are commonly labeled "Peeping Toms", which originates from the Lady Godiva legend.
(source: wikipedia, encyclopedia)