A gravimetric fluctuation is a localized and nonlinear variation in gravimetric fields and in the concentration of graviton subatomic particles.

On Stardate 43625.2, the sensors of the USS Enterprise-D picked up unusual gravimetric fluctuations, like that of a temporal displacement vortex, but there was no discernible event horizon, center or outer edge. Also, navigational subsystems were unable to give coordinates on the object. (TNG: "Yesterday's Enterprise")

In fact, this event occurred when the starship USS Enterprise-C, lost for 22 years, came through a temporal rift. After the return of the Enterprise-C to their own timeline, everyone was back at their normal stations and left no direct evidence of the temporal upheaval. (TNG: "Yesterday's Enterprise")