1. To allow a system to come to equilibrium with its surroundings. For example, a borehole gravimeter may have to rest at the bottom of a deep well bore for a period of time to allow it to adjust to the temperature and pressure regimen before meaningful readings can be obtained from it. 2. Time between injecting heat into a formation and producing fluids from it, to allow the heat to change the viscosity so that the fluids flow more readily.