Belief is acceptance that a statement is true or that something exists. A feeling of being sure that someone or something exists or that something is true. (Merriam-Webster dictionary)

Belief is the state of mind in which a person thinks something to be the case, with or without there being empirical evidence to prove that something is the case with factual certainty. Another way of defining belief sees it as a mental representation of an attitude positively oriented towards the likelihood of something being true. (Schwitzgebel, Eric (2006), "Belief", in Zalta, Edward, Stanford Encyclopedia of Philosophy, Stanford, CA:The Metaphysics Research Lab, retrieved 2008-09-19)

Belief is any sequence of steps, statements, or demonstrations leading to a valid conclusion; any perception, cognition, or emotion that the brain assumes, consciously or unconsciously, to be true. “Perception” refers to information we receive about ourselves and the world through the senses. Cognition represents a different level of processing within the brain and includes all the abstract conceptual processes that the brain uses to organize and make sense of our perceptions. (Newberg, Andrew, MD and Mark Robert Waldman. Why We Believe What We Believe. P 20-21. NY:Free Press, 2006)