The most common definition of belief is taken from analytical philosophy, which understands it as a proposition that is considered as true. Such a broad definition is ambiguous for some fields of empirical research, like psychology, which deals with the mental state of the believer when holding the belief. This article aims to reach an operationalization of beliefs to pinpoint their distinctive features with respect to similar concepts (knowledge, opinion, preference, perception or prediction, for instance). We summarize the most influential interpretations of belief in psychology and psychiatry, which are mainly based on Immanuel Kant and analytical philosophy. We also expose the problem that arises from putting the mental states of beliefs, knowledge, opinions and preferences in the same bag. Our proposal is that a belief is: (1) a proposition that is taken to be true; and (2) which the subject is willing to hold even if irrefutable evidence were hypothetically argued against it. We introduce a narrower interpretation to reliably discriminate the mental state of believing, which is intended to be applied to empirical psychology, neuroscience, behavioral economics, sociology and related sciences.