What term describes a person who rules in place of someone unable to rule?

A regent is precisely defined as an individual appointed to govern or rule on behalf of a monarch or leader who is unable to perform their duties, often due to reasons such as minority, absence, or incapacity. This role is significant in various historical contexts, where a regent temporarily assumes the powers and responsibilities typically held by the ruler until the rightful leader can resume their position.

In contrast, a monarch refers to the sovereign leader of a monarchy, and while they can be absent, they are not ruling in place of another. A viceroy typically refers to a governor exercising authority on behalf of a sovereign in a colonial context and does not necessarily act in place of a monarch. A senator, on the other hand, is a member of a legislative body and does not have ruling powers akin to those of a regent. Thus, the term that directly captures the essence of ruling in place of someone unable to do so is indeed regent.