A temporal differential (or space-time differential) is a phenomenon where time passes at different rates in two separate locations.
A planet with a tachyon core, discovered by Voyager in 2376, experienced a temporal differential of approximately one second outside per 16 hours inside. When the inhabitants of the planet attempted to explore Voyager, the transition between their time frame and Voyager's proved debilitating. The planet's civilization eventually developed temporal compensators, which allowed them to briefly interact with the normal space-time continuum. (VOY: "Blink of an Eye")