Personification of knowledge (Greek Επιστημη, Episteme) in Celsus Library in Ephesos, Turkey.

Knowledge is what is known. Like the related concepts truth, belief, and wisdom, there is no single definition of knowledge on which scholars agree, but rather numerous theories and continued debate about the nature of knowledge.

Knowledge acquisition involves complex cognitive processes: perception, learning, communication, association, and reasoning. The term knowledge is also used to mean the confident understanding of a subject, potentially with the ability to use it for a specific purpose.