ELI5 Gaslighting - what exactly does it mean?

Where does the term come from? Why is the definition so hard to pin down? What happened in the 1940s film starring Ingrid Bergman? When did gaslighting re-enter popular culture in recent times? Why is the term often misused?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: pwa09, berael, sublimeecto1a, e-snap, nobutthanksanyway, bodaciousvermin, axolotlsaredangerous, encogneeto and diaperedwoman

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