Déjà vu is a French term that translates literally as "already seen". It has been adopted into English to describe the experience of seeing something that seems familiar but where the reason for this familiarity is not apparent. It is also used to describe (for example) art that purports to be avant garde but just reworks old ideas.

You imply that there is something mysterious about the feeling. It seems no more mysterious, to me, than forgetting the bon mot that one is struggling to remember. Sometimes a little aide-mémoire can help.

Au revoir,

Pete