Consciousness remains an elusive concept due to the dif®culty to de®ne what has been regarded for many years as a subjective experience, therefore irrelevant for scienti®c study. Recent development in this ®eld of research has allowed to provide some new insight to a possible way to de®ne consciousness. Going through the extensive literature in this domain, several perspectives are proposed to de®ne this concept. 1) Consciousness and Attention may not re ̄ect the same process. 2) Consciousness during wake and sleep may not involve the same mechanisms. 3) Besides physiological states of consciousness, human beings can experience modi®ed states of consciousness either by self- training transcendental meditation, hypnosis, etc.) or by drug intake hallucinogens, anaesthetics, etc.). Altogether, we address the question of a more precise terminology, given the theoretical weight words can convey. To this respect, we propose different de®nitions for concepts like consciousness, vigilance, arousal and alertness as candidates to separate functional entities.