A power signature was a distinctive energy emission unique to technologies of individual space-faring species.
In 2151, Subcommander T'Pol analyzed the power signature of an approaching vessel, and was able to determine it belonged to the same species who had murdered the crew of an Axanar cargo ship, by comparing the power signature to the bio-pump that was being used to drain the murdered crew of triglobulin. (ENT: "Fight or Flight")
In 2154, the crew of Enterprise NX-01 was able to determine that an apparent Tellarite cruiser that attacked an Andorian ship, and an Andorian ship that attacked Enterprise, were in fact, the same ship, by analyzing its power signature. (ENT: "Babel One")