Déjà vu was a Terran term (from the French language for "already seen") used to describe the feeling of having previously experienced something that was being experienced for the first time. (ENT: "Future Tense") The corresponding term in Klingonese was "nIb'poH." (TNG: "Cause and Effect")
Tuvok referred to the phenomenon as "paradoxical state-dependent associative phenomenon." In 2373, Lieutenant Tuvok said that Ensign Harry Kim was experiencing it when he claimed that the USS Voyager had visited the star system before where the Taresian homeworld is located. (VOY: "Favorite Son")
Déjà vu is also associated with the phenomenon of alternate realities. When trapped in a temporal causality loop, an independent fragment of time, a period of time of the entire universe repeats itself. Eventually memories of previous loops imprint themselves to persons inside the loop, manifesting at first in the form of déjà vu. Such an event occurred in 2368, when the explosion of the warp core of the USS Enterprise-D next to a highly localized temporal distortion triggered a temporal causality loop. (TNG: "Cause and Effect")
Harry Kim also felt an alternate reality might explain his intense sense of déjà vu regarding a region of space in the Delta Quadrant until a retrovirus was found to be the actual cause of the unexplained memories. (VOY: "Favorite Son")