A belief was the conviction or acceptance of something by an individual that it existed or was true, especially without proof. Beliefs affected the morals and values of an individual. Beliefs were mostly related to religion.
A set of beliefs was known as a belief system.
In 2266, after informing the crew of the USS Enterprise that they would be destroyed, Balok, the commander of the Fesarius, said, "We make assumption you have a deity or deities or some such beliefs which comfort you." (TOS: "The Corbomite Maneuver")