What phase of Action Potential (AP) exactly defines the Relative Refractory Period?
Neither is an accurate definition; both are correct and not conflicting if you interpret &quot;corresponding&quot; to mean &quot;typically/approximately corresponding&quot;.
The absolute refractory period is any time in which you cannot evoke another action potential no matter the strength of stimulus, due to an action potential in progress/that has just occurred.
The relative refractory period is any time in which you can trigger an action potential but need a stronger stimulus than normal.
Neither of these definitions refers to specific phases of the action potential in terms of voltage or change in voltage. Yes, typically the absolute period includes the ascending and descending phases of the AP, but there's no reason it can't last longer depending on the gating properties of the sodium channels. Similarly, the relative refractory period starts with the end of the absolute refractory period and continues through the hyperpolarization phase, but there is no reason it cannot end sooner or later, again depending on gating properties.