1. An abnormal inability to comprehend, or to understand: "Some people have acatalepsia which is a form of mental deficiency."
2. In philosophy, an incomprehensibleness, or the impossibility of comprehending or conceiving anything: "Acatalepsia is the incomprehensibility of things; such as, the doctrine held by the ancient Skeptic philosophers, that human knowledge cannot be certain, but only the possibility that something will happen."