Technosignature

A technosignature is observable evidence of an alien civilization's presence, such as industrial waste products in the atmosphere of a planet, or spacecraft found to have visited a particular astronomical target. Many can be detected across interstellar distances.
Xenologists separate them into two types: active technosignatures, a civilization making its presence known to others through radio beacons or space-based megastructures deliberately used to signal the intelligence of their creators, and passive signatures which are byproducts of other activities like mining or power generation.
If the civilization which produced the signature has collapsed or otherwise ceased to exist, it is known as a necrosignature.