Stray light is light that reaches a telescope's sensors (or plates) other than that intended. Some sources of stray light are due to the physical design of the telescope or instrument, i.e., reflections off surfaces, which can be classified as ghost images, such as weak additional images of a different size produced by reflections off lens surfaces, and scattered light, leaking in from other directions or misdirected, such as via through reflections off internal parts. Light pollution can also be thought of as a source.
The design of a telescope and its instruments requires attention to avoiding the problem and stray light mitigation is efforts to reduce it. In some cases, this involves designing in extra structure simply to block the unwanted light (baffles).
The term stray light is commonly used for optical telescopes but analogous issues undoubtedly affect telescopes for any portion of the EMR spectrum.