A space-time scanner was a small, hand-held device which Temporal Agent Daniels used. It operated by projecting what he termed a "Temporal Observatory".
In 2151, Silik attempted to steal the scanner and deliver it to the Suliban Cabal's mysterious benefactor, which would have provided his faction an advantage in the Temporal Cold War. Captain Archer destroyed the device with a phase-pistol, rather than let it fall into the wrong hands. (ENT: "Cold Front")
The term "space-time scanner" wasn't used in canon and comes from the script of "Cold Front". That script described it as "
a hand-held, futuristic device. (What we'll learn is a space-time scanner.)"