Knowledge is a familiarity with someone or something, that can include descriptions, facts, information, and/or skills acquired through experience or education. It can refer to both the theoretical or practical understanding of a subject. It can be implicit (as with practical skill or expertise) or explicit (as with the theoretical understanding of a subject); and it can be more or less formal or systematic. In philosophy, the study of knowledge is called epistemology, and the philosopher Plato famously defined knowledge as "justified true belief." There is however no single agreed upon definition of knowledge, and there are numerous theories to explain it.

Knowledge acquisition involves complex cognitive processes: perception, learning, communication, association and reasoning; while knowledge is also said to be related to the capacity of acknowledgment in human beings.