A spurious emission is any radio frequency

Radio frequency

Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 not deliberately created or transmitted, especially in a device which normally does create other frequencies. A harmonic

Harmonic

A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...

 or other signal outside a transmitter

Transmitter

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

's assigned channel would be considered a spurious emission.

From ITU, 1.145 Spurious emission: Emission on a frequency or frequencies which are outside the
necessary bandwidth and the level of which may be reduced without affecting the
corresponding transmission of information. Spurious emissions include harmonic
emissions, parasitic emissions, intermodulation products and frequency conversion
products but exclude out-of-band emissions.