The password itself is not stored in AD in decryptable form. As I understand it what is stored is a hash of the password and the date/time when the password was set. When a user logging on enters the password that value and the date/time when the password was last set are used to re-calculate the stored hash. if this is the same as the stored hash, the assumption is that the user entered a valid password.
Full Definition of CREDENTIAL. 1. : something that gives a title to credit or confidence; also: qualification 3a. 2. plural: testimonials or certified documents showing that a person is entitled to credit or has a right to exercise official power.
A diploma (from Greek Î´Î¯ÏÎ»ÏÂµÎ± dÃ­plÅma, meaning folded paper) is a certificate or deed issued by an educational institution, such as a college or university, that testifies that the recipient has successfully completed a particular course of study or confers an academic degree.