What term defines the act of filling up again something that has been used up?

Replenishment refers specifically to the act of restoring a supply that has diminished or been used up. This term is commonly used in various contexts, such as in natural resource management, inventory control, and waste management, where it denotes the process of ensuring that a resource is brought back to its original or required level after it has been depleted.

In the context of the question, replenishment fits best because it encompasses the idea of not only refilling but also ensuring that sufficient quantities are present to meet demand. The focus of replenishment is on maintaining adequate supplies, which aligns with the notion of effective management of resources.

Other terms, while related, do not capture this specific meaning. For instance, restoration typically implies returning something to its original condition rather than simply refilling. Recuperation usually refers to recovery from an illness or a loss, involving a process of regaining strength or health, rather than the act of filling up a resource. Refill also denotes adding to something that has decreased but lacks the broader connotation of maintaining a sustainable supply that replenishment entails.