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1.
When the sight of a person or thing brings back a vague memory and/or feeling of a dream you had the previous night. More likely than not, the person experiencing the dreamja vu cannot remember any additional details of the dream, creating a deja vu-like feeling of "can't put my finger on it, but I remember it." Often causes interactions with the person or thing to take on a surreal feeling for the person experiencing dreamja vu.

This is not to be confused with suddenly remembering the entirety of a dream or a dream coming true.
Person 1: "Oh hey, Jennifer! ... Whoa, dreamja vu."
Person 2: "What?"
Person 1: "Oh, I think I had a dream about you last night."
Person 2: "What happened in it?"
Person 1: "No clue."
by Psycho Logic March 16, 2011
 
2.
when you realize as you are dreaming that you have had the same dream before
OMG this train is gonna run me over!!! ...Oh dream ja vu!!
by sillyna November 11, 2010