Final answer:
The period during which a second action potential requires a stronger stimulus intensity is called the relative refractory period.
Step-by-step explanation:
The time during an action potential, when a second action potential can only be generated with an increased stimulus intensity, is known as the relative refractory period. This period follows the absolute refractory period, during which no second action potential can be initiated regardless of the strength of the stimulus. It is during the relative refractory period that the voltage-gated K+ channels are not yet closed and the membrane potential is returning to its resting state, making it possible, yet challenging, to trigger another action potential.