***Silly Question Warning***
I did a search and can't find the answer to the following probably silly question: What is the actual musical theatre definition of a "belter" or "belt?" Is it volume? ...range? ...both? ...or, something else (besides an accessory to keep your pants from falling down!)

Thank you, thank you, thank you

Singers can chime in here with a more technical explanation, but i think it is using the chest voice in the higher ranges to produce a bigger sound.

Don't know if you got your answer yet or not, but your belt voice is the voice that blends both your chest voice(lower register) and your head voice (upper register)creating a strong sound. There are those innately gifted people who can do this naturally, but it can sometimes be learned with proper training.

Ethel Merman was a "belter".

Thanks so much clarifying that!