In the context of password security, what does 'password complexity' refer to?

Password complexity refers to the various elements that make a password difficult to guess or crack. It is primarily concerned with the variety and type of characters used in the password. A complex password typically includes a combination of uppercase letters, lowercase letters, numbers, and special characters. This mixture of characters increases the number of possible combinations, thus enhancing security.

An example of a complex password would be "P@ssw0rd123!" which uses uppercase letters, lowercase letters, numbers, and symbols, making it significantly harder for attackers to guess or use brute-force methods to crack.

The other options, while related to password security, do not address the specific definition of complexity. The length of a password is important for security but is considered a separate aspect of password strength. The history of previously used passwords is relevant for preventing reuse but does not define complexity itself. Similarly, the frequency of password changes can contribute to security but does not alter the inherent complexity of a password.