An uppercase letter in a password refers to a capital letter, which is a larger and more prominent version of the standard, smaller lowercase letters. Passwords should contain three of the four character types: uppercase letters (A-Z), lowercase letters (a-z), numbers (0-9), and symbols (~`!@#$%^&*()_-+={. When calculating the number of character types, upper-case letters used as the first character and digits used as the last character of a password are not counted. For example, the password Atu157! will not work because it starts with the upper-case A, but password aTu157! will pass the quality check. Therefore, it is important to include at least one uppercase letter in your password to make it more secure.