Simple Definition:

An apocalypse is a catastrophic event or the end of the world, often associated with widespread destruction and chaos. (Catastrophic event: a disastrous occurrence with severe consequences)

Examples:

• The apocalypse in the movie was depicted as a global pandemic that wiped out most of humanity. (Pandemic: an outbreak of a disease affecting a large number of people)
• Some people believe that climate change could lead to an environmental apocalypse, causing irreversible damage to the planet. (Irreversible: impossible to reverse or undo)

Very Simple Definition:

An apocalypse is a really bad thing that happens and everything gets destroyed.

Examples:

• The world in the movie was destroyed in an apocalypse where buildings collapsed and people ran away.
• In the book, there was an apocalypse where all the food disappeared and people were hungry.