Ability cooldown

Besides waiting for energy/stamina to regenerate and the delay in performing the ability, skill or spell, player must also wait an arbitrary amount of time between finishing using it once and using it again. The cooldown period is often unexplained, has no effect on using other abilities, skills or spells, and mostly serves to force players to use variety of abilities, skills or spells instead of spamming the same one over and over again.

In short: it's a crude way to enforce variety.

On related note: In some systems this is quite explainable, such as if you consider battle tank's cannon use to be an ability. The cooldown period is essentially the time it takes for the cannon crew to reload the weapon or in case of automated loading system, for that system to perform its function. But then again, most abilities in games tend to be something comparable to jumping or swinging your arm, so it makes little sense.