Hysteresis is the dependence of the state of a system on its history (i.e., how a system acts "later" in time dependent on its state some unit of time before). In medicine, it often refers to lung physiology. Lung hysteresis is evident when observing the compliance of a lung on inspiration versus expiration. The difference in compliance (Δvolume/Δpressure) is due to the additional energy required to overcome surface tension forces during inspiration to recruit and inflate additional alveoli.

More formally and outside the context of medicine, hysteresis describes a property of a system such that an output value is not a strict function of the corresponding input, but also incorporates some lag, delay, or history dependence, and in particular when the response for a decrease in the input variable is different from the response for an increase. For example, a thermostat with a nominal setpoint of 75° might switch the controlled heat source on when the temperature drops below 74°, and off when it rises above 76°.