Bistability

Something that is bistable can be resting in two states. In physics, for an ensemble of particles, the bistability comes from the fact that its free energy has three critical points. Two of them are minima and the last is a maximum. By mathematical arguments, the maximum must lie between the two minima. By default, the system state will be in either of the minima states, because that corresponds to the state of lowest energy. The maximum can be visualised as a barrier.

A transition between the state of minimal free energy needs some form of activation energy to penetrate the barrier (compare activation energy and Arrhenius equation for the chemical case.) After the barrier has been reached, the system will relax into the next state of lowest energy again. The time it takes is usually attributed the relaxation time. (There might be uncertainty as to which state will be the new one, but it is often well defined in the situation.)

Bistability and memory
In 2004, Sara Solla et al. developed a computer model of short-term memory constructed around a small-world network. This model successfully demonstrated bistability, a property thought to be important in memory storage. The bistability appears to be the result of recurrent self-sustaining loops of activity after an activating pulse is given. A second pulse would turn off the system. Hence, the pulses switch the system between its bistable states.