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" )