What exactly is déjà vu?

James M. Lampinen, assistant professor of psychology at the University of Arkansas, supplies this answer:

                 Most people experience déjà vu—the feeling that an entire event has happened before, despite the knowledge that it is unique. We don’t yet have a definitive answer about what produces déjà vu, but several theories have been advanced.

                 One early theory, proposed by Sigmund Freud, is that déjà vu takes place when a person is spontaneously reminded of an unconscious fantasy. In 1990 Herman Sno, a psychiatrist at Hospital de Heel in Zaandam, the Netherlands, suggested that memories are stored in a format similar to holograms. Unlike a photograph, each section of a hologram contains all the information needed to reproduce the entire picture. But the smaller the fragment, the fuzzier the resultant image. According to Sno, déjà vu occurs when some small detail in one’s current situation closely matches a memory fragment, conjuring up a blurry image of that former experience.

                  Déjà vu can also be explained in terms of what psychologists call global matching models. A situation may seem familiar either because it is similar to a single event stored in memory or because it is moderately similar to a large number of stored events. For instance, imagine you are shown pictures of various people in my family. Afterward, you happen to bump into me and think, “Hey, that guy looks familiar.” Although nobody in my family looks just like me, they all look somewhat like me, and according to global matching models the similarity tends to summate.

                  Progress toward understanding déjà vu has also been made in cognitive psychology and the neurosciences. Researchers have distinguished between two types of memories. Some are based on conscious recollection; for example, most of us can consciously recall our first kiss. Other memories, such as those stimulated when we meet someone we seem to recognize but can’t quite place, are based on familiarity. Researchers believe that conscious recollection is mediated by the prefrontal cortex and the hippocampus at the front of the brain, whereas the part housed behind it, which includes the parahippocampal gyrus and its cortical connections, mediates feelings of familiarity. Josef Spatt of the NKH Rosenhügel in Vienna, Austria, has argued that déjà vu experiences occur when the parahippocampal gyrus and associated areas become temporarily activated in the presence of normal functioning in the prefrontal cortex and hippocampus, producing a strong feeling of familiarity but without the experience of conscious recollection.

As you can tell, this is an area still ripe for research.