Named after the famous Nelson Mandela this conspiracy theory Is when a large group of people remember something being a certain way, but when people go back and look at it, it has changed or is gone in this universe with no reasoning other than "it's always been like that"

Two popular theories on the Mandela effect are:
Time Travel- Some believe that times travels have come back to the past and changed one little thing that ended up changing many other things kinda like the butterfly effect Think about it the future has already happened so even if time travel is say 1,000 years in the future it has already been done which means time travelers are going back to the past which is our present and changing little things.

Parallel Universe- Others believe that sometime in the last decade or two the world went through a black hole or that our "original world" ended and now we are in this Parallel Universe where something changed in the process of switching or going through the black hole. (most think in the years 2000 or 2012 this happened cause many thought the world was gong to end)
Kayla: Do you remember those books we used to read with the family of bears? The Berenstein Bears.
Danielle: MANDELA EFFECT! It has supposedly been the BerenstAin Bears the whole time!
Kayla: No way! I remember that vividly as a kid!
Danielle: Same here!
by GameofThronesjunkie December 27, 2016
A brain infection that causes a person suffering from it to be physically incapable of admitting a mistake. Saying phrases such as "I was wrong" or "I must have misremembered" create an acute sensation of pain, so the sufferer will do everything they can to avoid speaking those words. A common symptom is the invention of fanciful theories about a Parallel Universe and/or Time Travel to explain away memory lapses. This is a communicable disease. As of yet a cure has not been discovered, though taking a break from one's smart phone and/or computer and taking a walk outside does seem to soften the severity of the symptoms somewhat.
Billy: Yo, what's up with Jimmy? Have you noticed that he can never admit when he's wrong about something, even when it's super obvious?
Zachariah: Don't be too hard on him, Billy, he has the Mandela Effect. His brain is all messed up.
Billy: Ouch, poor bastard! I guess I'm going to have to avoid him until he gets better -- the last thing I need right now is to catch that too.
Zachariah: Smart idea.
by Brain from Terra Incognita January 6, 2017
A man made conspiracy theory used by fruity arse niggas that have very awful poor memory but blame it on a parallel universe or similar theories.
Hey Johno Fuck I'm positive I thought "Froot Loops" was spelt "Fruit Loops" it being spelt "Froot Loops" is crazy as fuck cunt - Nah mate you just lost the fucking plot it's always been Froot Loops the Mandela Effect is fake cunt"
by TheOnlyUrbanTruth March 7, 2017
Contrary to popular belief. The Mandela Effect has nothing to do with time travel or multiple alternate universes colliding.

The Mandela Effect is the recollection of events that never occured. (eg: Shazaam featuring Sinbad. It was actually Kazaam featuring Shaq.)

The name of the phenomena is derived from the belief that mandela died in 1991 in prison.
"Remember Sinbad in Shazaam? No. It never existed. It's the Mandela Effect"
by Jon Jonns July 31, 2017
Is where a bunch of people remember something clearly that actually never really happened in this alternate reality that we are now in
Derek :Luke I'm your father....
Damon :dude are you stupid it's No I'm your father.
Derek: wow the mandela-effect runs deep
by IlluminatFightR November 14, 2016
An perfect example of how dumb, forgetful and easily persuaded humans are.

I could say, "The word 'carcass' never existed," and everyone would start yelling "MANDELA EFFECT!"
Amy: Want a rice crispie?
Barb: It's rice KRISPie.
Amy: No no, Rice Krispie is a name of a company, rice crisp-
Amy: *rolls eyes*
by snafu love November 22, 2016
Historian: "You are eating a jar of Jif"
Pothead: "No, this is Jiffy"
Historian: "Ohhh, Mandela effect."