What is the science behind déjà vu?

white woman with blonde hair pictured walking down a city sidewalk at night with her phone illuminating her face. She's looking up at the surrounding scene as if confused

You're walking somewhere you've never been before and suddenly feel as though you've strolled down the same road already. You're experiencing the well-known phenomenon déjà vu — but what is déjà vu, really, and why does the strange feeling happen?

Déjà vu is a French expression meaning "already seen," which was first used in 1876 by French philosopher Émile Boirac in a letter to his book editor, and later in his published book "The Psychology of the Future" (Keagan Paul, 1918).