1. varying degrees of alertness, arousal, or vigilance, ranging from the degree experienced in normal wakefulness, to that experienced in unfocused wakefulness, drowsiness, light sleep, or slow-wave (deep) sleep, to that experienced minimally or not at all, as in a minimally conscious state, vegetative state, and coma. Levels of consciousness are defined both behaviorally and by electroencephaolography and other kinds of brain imaging.

2. in certain mystical philosophies, states of pure consciousness that may involve changes in perception, feeling, and thought and that may occur either spontaneously or as a result of practices such as yoga, meditation, breathing techniques, and fasting.