This Paraphilia is characterized by sexual fantasies, urges, or behaviors involving observing an unknowing and non-consenting person, usually unclothed and/or engaged in sexual activity, to produce sexual excitement.
Diagnostic criteria for 302.82 Voyeurism
A. Over a period of at least 6 months, recurrent, intense sexually arousing fantasies, sexual urges, or behaviors involving the act of observing an unsuspecting person who is naked, in the process of disrobing, or engaging in sexual activity.
B. The person has acted on these urges, or the sexual urges or fantasies cause marked distress or interpersonal difficulty.
from DSM IV-TR