voy·eur 
(voi-yûr)
(voi-yûr)n.
1. Psychology A person who derives sexual gratification from covert observation of an unsuspecting person who is naked, undressing, or engaging in sexual activity.
2. An enthusiastic observer of sordid or sensational subjects.
[French, from Old French, one who lies in wait, from voir, to see, from Latin vidēre, to see; see weid- in the Appendix of Indo-European roots.]
voy·eurism n.
voy′eur·istic adj.
voy′eur·isti·cal·ly adv.