Different way to refer to a 'lowercase' letter?

We also refer to them as minuscules, though this is less common. Calling a letter small, lower-case or lowercase is clear and concise, and you should go by those.

Personally, I don't know that I've heard majuscules or minuscules ever used within my hearing discussing matters of typography or written language.

I tend to think lowercase is by far the most prominent word to describe those characters, however, if referring to a single letter, it may also be considered uncapitalized.

Yes, “minuscule” and “majuscule” sum it up nicely.

Educated under the British system over 40 years ago, I was taught that the small letters were referred to as common letters, while the large letters were referred to as capital letters. Common letters does not seem to be a designation used in the USA.