Representation of consciousness from the seventeenth century by Robert Fludd, an English Paracelsian physician
Consciousness is the state or quality of awareness, or, of being aware of an external object or something within oneself. It has been defined variously in terms of sentience, awareness, qualia, subjectivity, the ability to experience or to feel, wakefulness, having a sense of selfhood or soul, the fact that there is something "that it is like" to "have" or "be" it, and the executive control system of the mind. In contemporary philosophy its definition is often hinted at via the logical possibility of its absence, the philosophical zombie, which is defined as a being whose behavior and function are identical to one's own yet there is "no-one in there" experiencing it.
Despite the difficulty in definition, many philosophers believe that there is a broadly shared underlying intuition about what consciousness is. As Max Velmans and Susan Schneider wrote in The Blackwell Companion to Consciousness: "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."
Western philosophers, since the time of Descartes and Locke, have struggled to comprehend the nature of consciousness and identify its essential properties. Issues of concern in the philosophy of consciousness include whether the concept is fundamentally coherent; whether consciousness can ever be explained mechanistically; whether non-human consciousness exists and if so how can it be recognized; how consciousness relates to language; whether consciousness can be understood in a way that does not require a dualistic distinction between mental and physical states or properties; and whether it may ever be possible for computing machines like computers or robots to be conscious, a topic studied in the field of artificial intelligence.
Thanks to developments in technology over the past few decades, consciousness has become a significant topic of interdisciplinary research in cognitive science, with significant contributions from fields such as psychology, anthropology,neuropsychology and neuroscience. The primary focus is on understanding what it means biologically and psychologically for information to be present in consciousness—that is, on determining the neural and psychological correlates of consciousness. The majority of experimental studies assess consciousness in humans by asking subjects for a verbal report of their experiences (e.g., "tell me if you notice anything when I do this"). Issues of interest include phenomena such as subliminal perception, blindsight, denial of impairment, and altered states of consciousness produced by alcohol and other drugs, or spiritual or meditative techniques.
In medicine, consciousness is assessed by observing a patient's arousal and responsiveness, and can be seen as a continuum of states ranging from full alertness and comprehension, through disorientation, delirium, loss of meaningful communication, and finally loss of movement in response to painful stimuli. Issues of practical concern include how the presence of consciousness can be assessed in severely ill, comatose, or anesthetized people, and how to treat conditions in which consciousness is impaired or disrupted. The degree of consciousness is measured by standardized behavior observation scales such as the Glasgow Coma Scale.
John Locke, British philosopher active in the 17th century
The origin of the modern concept of consciousness is often attributed to John Locke's Essay Concerning Human Understanding, published in 1690. Locke defined consciousness as "the perception of what passes in a man's own mind". His essay influenced the 18th-century view of consciousness, and his definition appeared in Samuel Johnson's celebrated Dictionary (1755). "Consciousness" (French: conscience) is also defined in the 1753 volume of Diderot and d'Alembert's Encyclopédie, as "the opinion or internal feeling that we ourselves have from what we do." 
The earliest English language uses of "conscious" and "consciousness" date back, however, to the 1500s. The English word "conscious" originally derived from the Latin conscius (con- "together" and scio "to know"), but the Latin word did not have the same meaning as our word—it meant "knowing with", in other words "having joint or common knowledge with another". There were, however, many occurrences in Latin writings of the phrase conscius sibi, which translates literally as "knowing with oneself", or in other words "sharing knowledge with oneself about something". This phrase had the figurative meaning of "knowing that one knows", as the modern English word "conscious" does. In its earliest uses in the 1500s, the English word "conscious" retained the meaning of the Latin conscius. For example, Thomas Hobbes in Leviathan wrote: "Where two, or more men, know of one and the same fact, they are said to be Conscious of it one to another." The Latin phrase conscius sibi, whose meaning was more closely related to the current concept of consciousness, was rendered in English as "conscious to oneself" or "conscious unto oneself". For example, Archbishop Ussher wrote in 1613 of "being so conscious unto myself of my great weakness". Locke's definition from 1690 illustrates that a gradual shift in meaning had taken place.
A related word was conscientia, which primarily means moralconscience. In the literal sense, "conscientia" means knowledge-with, that is, shared knowledge. The word first appears in Latin juridical texts by writers such as Cicero. Here, conscientia is the knowledge that a witness has of the deed of someone else.René Descartes (1596–1650) is generally taken to be the first philosopher to use conscientia in a way that does not fit this traditional meaning. Descartes used conscientia the way modern speakers would use "conscience". In Search after Truth (Regulæ ad directionem ingenii ut et inquisitio veritatis per lumen naturale, Amsterdam 1701) he says "conscience or internal testimony" (conscientiâ, vel interno testimonio).