In a password, an uppercase letter is a letter that is written in the upper case, while a lowercase letter is a letter that is written in the lower case. For example, the letter "A" is an uppercase letter, while the letter "a" is a lowercase letter. Many websites require users to include both uppercase and lowercase letters in their passwords to make them more secure. It is important to note that uppercase and lowercase letters are treated as different characters, so "A" and "a" are not the same character in a password.