A(n) period is a brief time following a nerve impulse when the membrane is unresponsive to an ordinary stimulus.

The term you are looking for is "refractory period."

The refractory period is a crucial phase in the action potential of a neuron, during which the neuron is either completely unresponsive (absolute refractory period) or less responsive to stimuli (relative refractory period). After an action potential has occurred, potassium ions flow out of the neuron to help return the membrane potential to its resting state, creating a window of time during which the neuron cannot fire again or requires a stronger stimulus to do so. This period is essential for ensuring that impulses travel in one direction along the nerve and contributes to the overall functioning of the nervous system.