There is great confusion about the concept of impulsivity despite its important clinical use. Limits with others symptoms or behaviors, like aggressiveness, are imprecise, and we find no general agreement on the definition of impulsivity in the present literature. From the point of view of neurosciences, adequate identification and measurement of a symptom as impulsivity could increase the validity of syndromic diagnoses, improve the development of animal models and genetic studies, and improve the treatment of several psychiatric disorders. Based on this perspective, the overall goals of this review article are: a) to establish the historical evolution of term, concept and behavior that we presently identify as impulsivity, starting from the classical psychopathology of the will; b) to review several concepts of impulsivity described in the scientific literature, and c) to provide basic guidelines for an operative definition, just as some authors have proposed, to create research lines and understand various discoveries that basic sciences have contributed.