The person in this role is 'acting' as the role...But their rank is usually a grade less.

A lieutenant (abbreviated Lt., LT., Lieut. and LEUT.) is a junior commissioned officer in many nations' armed forces, fire service, or law enforcement.

The meaning of lieutenant differs in different military formations (see comparative military ranks), but is often subdivided into senior (first lieutenant) and junior (second lieutenant) ranks. In navies it is often equivalent to the army rank of captain; it may also indicate a particular post rather than a rank. The rank is also used in fire services, emergency medical services, security services and police forces.

Lieutenant may also appear as part of a title used in various other organizations with a codified command structure. It often designates someone who is "second-in-command," and as such, may precede the name of the rank directly above it. For example, a "lieutenant master" is likely to be second-in-command to the "master" in an organization using both ranks.

Political uses include lieutenant governor in various governments, and Quebec lieutenant in Canadian politics. In the United Kingdom, a Lord Lieutenant is the sovereign's representative in a county or lieutenancy area, while a Deputy Lieutenant is one the Lord Lieutenant's deputies.