1. A hypothesis stating that Earth's biota constitute a single interconnected system that affects or determines the physical and chemical conditions within the biosphere, including such conditions as global temperatures, the composition of the atmosphere, and the salinity of seawater.

2. Any of various related hypotheses stating that this system is self-regulating, as through feedback loops, or that it constitutes a living organism, in either case acting to maintain stable conditions that are optimal for the continuation of life.