A password is a secret word or string of characters that is used for authentication, to prove identity or gain access to a resource (Example: An access code is a type of password). The password must be kept secret from those not allowed access.

The use of passwords is known to be ancient. Sentries would challenge those wishing to enter an area or approaching it to supply a password or watchword. Sentries would only allow a person or group to pass if they knew the password. In modern times, user names and passwords are commonly used by people during a log in process that controls access to protected computer operating systems, mobile phones, cable TV decoders, automated teller machines (ATMs), etc. A typical computer user may require passwords for many purposes: logging in to computer accounts, retrieving e-mail from servers, accessing programs, databases, networks, web sites, and even reading the morning newspaper online.

Despite the name, there is no need for passwords to be actual words; indeed passwords which are not actual words may be harder to guess, a desirable property. Some passwords are formed from multiple words and may more accurately be called a passphrase. The term passcode is sometimes used when the secret information is purely numeric, such as the personal identification number (PIN) commonly used for ATM access. Passwords are generally short enough to be easily memorized and typed.

For the purposes of more compellingly authenticating the identity of one computing device to another, passwords have significant disadvantages (they may be stolen, spoofed, forgotten, etc.) over authentications systems relying on cryptographic protocols which are more difficult to circumvent.

Easy to remember, hard to guess

The easier a password is for the owner to remember generally means it will be easy for a hacker to guess. Passwords which are difficult to remember will reduce the security of a system because (a) users might need to write down or electronically store the password, (b) users will need frequent password resets and (c) users are more likely to re-use the same password. Similarly, the more stringent requirements for password strength, e.g. "have a mix of uppercase and lowercase letters and digits" or "change it monthly", the greater the degree to which users will subvert the system.

In , Jeff Yan et al. examine the effect of advice given to users about a good choice of password. They find that passwords based on thinking of a phrase and taking the first letter of each word, are just as memorable as naively selected passwords, and just as hard to crack as randomly generated passwords. Combining two unrelated words is another good method. Having a personally designed "algorithm" for generating obscure passwords is another good method.

However, asking users to remember a password consisting of a “mix of uppercase and lowercase characters” is like asking them to remember a sequence of bits: hard to remember, and only a little bit harder to crack (e.g. only 128 times harder to crack for 7-letter passwords, less if the user simply capitalises the first letter). Asking users to use "both letters and digits" will often lead to easy-to-guess substitutions such as 'E' --> '3' and 'I' --> '1', substitutions which are well known to crackers. Similarly typing the password one keyboard row higher is a common trick known to crackers.