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.
Qualification(noun) the act of qualifying, or the condition of being qualified. Qualification(noun) that which qualifies; any natural endowment, or any acquirement, which fits a person for a place, office, or employment, or which enables him to sustian any character with success; an enabling quality or circumstance; requisite capacity or possession.