An unintentional radiator
is any device which creates radio frequency
energy within itself, which does not normally leave the device or its housing, unless it is damaged or malfunctioning.
A computer would be an excellent example, as all spurious emissions should normally be contained within the case. Likewise, radio receivers use an intermediate frequency which is detectable outside the radio -- the concept behind Arbitron's portable people meter.
See also: intentional radiator and incidental radiator.