1. the state of being conscious.

2. an organism’s awareness of something either internal or external to itself.

3. the waking state (see wakefulness).

4. in medicine and brain science, the distinctive electrical activity of the waking brain, as recorded via scalp electroencephalogram, that is commonly used to identify conscious states and their pathologies.

Beyond these succinct, in some cases everyday, senses of the term, there are intricate philosophical and research controversies over the concept of consciousness and multiple perspectives about its meaning. Broadly, these interpretations divide along two (although not always mutually exclusive) major lines: (a) those proposed by scholars on the basis of function or behavior (i.e., consciousness viewed “from the outside”—the observable organism); and (b) those proposed by scholars on the basis of experience or subjectivity (i.e., consciousness viewed “from the inside”—the mind). The former generally represents the reductionist or materialist perspective (see materialism), whereas the latter generally represents the immaterialist perspective (see immaterialism). For example, functional or behavioral interpretations tend to define consciousness in terms of physical, neurobiological, and cognitive processes, such as the ability to discriminate stimuli, to monitor internal states, to control behavior, and to respond to the environment. According particularly to this view, the contents of consciousness are assessed through their ability to be reported accurately and verifiably (see reportability), although recent brain imaging research suggests that brain indices of conscious contents may become available.

Experiential or subjective interpretations, however, tend to define consciousness in terms of mental imagery; intuition; subjective experience as related to sensations, perceptions, emotions, moods, and dreams; self-awareness; awareness of awareness itself and of the unity between the self and others and the physical world; stream of consciousness; and other aspects of private experience. According to this view, the contents of consciousness can be assessed to some extent by their reportability but must also, given their phenomenological nature, rely on introspection. See also altered state of consciousness; evolution of consciousness.