Answer:
When paleontologist refer to the "Big Five" they are referring to the big five mass extinction events that had happened on Earth.
Explanation:
The five events;
The Ordovician-Silurian extinction event
The (Late) Devonian extinction event
The Permian-Triassic extinction event
The Triassic-Jurassic extinction event
The Cretaceous-Tertiary extinction event
