system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the Gaia Theory?
gpt
Gaia Theory is simply that you can study the entire earth as if it were a single organism.  All life and non-biological substances on the planet react to each other, and evolved in such a way that they are impacted by each other.  As such, you can study large scale changed to the earth (both the biological and non-biological parts) as all part of a complex interworking system, the same way you might study one of the complex systems in humans (such as the Krebs Cycle).

At times it has be criticized because proponents have a tendency to act a little bit too much as if the Earth actually is a living organism, and it's sometimes used by people in a pseudoscience way, but there is good science around the theory.