Consciousness is variously defined as subjective experience, awareness, the ability to experience "feeling", wakefulness, or the executive control system of the mind. It is an umbrella term that may refer to a variety of mental phenomena. Although humans realize what everyday experiences are, consciousness refuses to be defined, philosophers note (e.g. John Searle in The Oxford Companion to Philosophy):

"Anything that we are aware of at a given moment forms part of our consciousness, making conscious experience at once the most familiar and most mysterious aspect of our lives." - Schneider and Velmans, 2007

Consciousness in medicine (e.g., anesthesiology) is assessed by observing a patient's alertness and responsiveness, and can be seen as a continuum of states ranging from alert, oriented to time and place, and communicative, through disorientation, then delirium, then loss of any meaningful communication, and ending with loss of movement in response to painful stimulation.

Consciousness in psychology and philosophy typically means something beyond what it means for anesthesiology, and may be said in many contexts to imply four characteristics: subjectivity, change, continuity, and selectivity. Philosopher Franz Brentano has suggested intentionality or aboutness (that consciousness is about something). However, within the philosophy of mind there is no consensus on whether intentionality is a requirement for consciousness.

Consciousness is the subject of much research in philosophy of mind, psychology, neuroscience, cognitive science, cognitive neuroscience and artificial intelligence. Issues of practical concern include how the presence of consciousness can be assessed in severely ill or comatose people; whether non-human consciousness exists and if so how it can be measured; at what point in fetal development consciousness begins; and whether computers can achieve a conscious state.

Source: http://en.wikipedia.org/wiki/Consciousness