1. Generally, any initial change, in response to a sudden step change in some factor, that is greater than the steady-state response to the new level of that factor; common in systems in which inertia or a time lag in negative feedback outweighs any damping that may be present. Changes in a negative direction are sometimes distinguished by the term undershoot, and the two may alternate in an oscillatory fashion, as in the transient oscillations of a pendulum when released from an initial displacement.
2. Momentary reversal of the membrane potential of a cell (inside becoming positive rather than negative relative to the outside) during an action potential; considered a form of overshoot because, before discovery of overshoot, excitation was thought merely to depolarise the membrane to zero transmembrane potential.