Defined as:
"The phenomenon where a group of people discover that
a global fact - one they feel they know to be true and have
specific personal memories for -
has apparently changed in the world around them."
So basicaly a Mandela effect is where you realise that something you knew to be a general fact of the world, no longer seems to be true.
The "effect" is named after a particularly common example: many people remember Nelson Mandela dying in prison in the 1980s, when in fact he survived long after his release.
Examples:
● Celebrity deaths that now didn't happen.
● Geographical locations which seem to have shifted.
● Movies, books whose details seem to have changed.
Given nature of the effect, 'evidence' inevitably takes the form of shared
personal memories;
so feel free to share yours in this thread if you have experienced something close to what this effect means! 
