What is the proper address for a Lieutenant Colonel?

The proper address for a Lieutenant Colonel is indeed Colonel. In military settings, it is customary to address officers in a way that reflects their rank, and "Colonel" is an appropriate term of respect for a Lieutenant Colonel. This is due to the fact that the rank of Lieutenant Colonel is subordinate to Colonel, but in formal contexts, such as ceremonies or official communications, using the higher title is standard practice as a mark of respect.

In addressing a Lieutenant Colonel, using "Colonel" acknowledges their position and authority, even though it does not denote their full rank. It's important to maintain this etiquette, as it fosters respect and recognition of the hierarchy within military culture.