There is no definition of the word "belief".Everybody decides for themselves what it means. You can't measure it, but this thing determines your life. It is the system of principles, laws, rules, feelings, emotions.This word rules the world. Majority of people believe and follow worldwide known religions. Even Atheism has "the belief in disbelief". It sounds strange but also has its own meaning, where involved psychology, philosophy, world history, science. But the most important thing is that they are all interconnected and form "belief". You can believe in God, you can believe in yourself, even you can believe in unmanageable chaos. There are a lot of ways how we can describe this word. When the noun "belief" becomes a verb "believe" - here arise and happen different things, they can be good and they can be bad. Your belief makes your life the way it is. Maybe, the Earth is still revolving around the Sun, because everybody, who falls asleep in this world, believes that morning will come, who knows?