lowercase letter
Specifically a letter in its lowercase form, as opposed to uppercase (大文字).
See also: 大文字
Please mix uppercase and lowercase letters in your password.
small letter
A letter written in a small size, or a letter that is physically small; less common than the lowercase sense.
He wrote densely in small letters in his notebook.
Please write that word in lowercase letters.
I converted all the characters to lowercase in the program.
This password is case-sensitive, distinguishing between uppercase and lowercase letters.