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 should be kept secret from those not allowed access.
Reference: Wikipedia http://en.wikipedia.org/wiki/Password, retreived 2011-04-25
More unconventional authentication methods that can be used in place of passwords are combinations of gestures and movements or the ability to correctly identify persons or objects (for example Facebook uses recognition from photos, see this article by Josh Constine, published Jul 26th, 2010).
Questions with this tag deal with creating, confirming, resetting and generally using passwords.