Equilibrant Force, which keeps any object motionless and acts on virtually every object in the world that is not moving Equilibrium figures of Earth and planets (physical geodesy) Equilibrium mode distribution, the state of fiber optic or waveguide transmission in which the propagation mode does not vary with distance along the fiber or changes in the launch mode Hydrostatic equilibrium, the state of a system in which compression due to gravity is balanced by a pressure gradient force Hyperbolic equilibrium point, a mathematical concept in physics Mechanical equilibrium, the state in which the sum of the forces, and torque, on each particle of the system is zero Radiative equilibrium, the state where the energy radiated is balanced by the energy absorbed Secular equilibrium, a state of radioactive elements in which the production rate of a daughter nucleus is balanced by its own decay rate Thermodynamic equilibrium, the state of a thermodynamic system which is in thermal, mechanical, and chemical equilibrium (see below). When it is said that a system is simply "in equilibrium", this is often what is meant.