I guess you could say is has been defined in a lot of different ways by a lot of different people, but it has not yet been concretely defined.
"Consciousness may involve thoughts, sensations, perceptions, moods, emotions, dreams, and self-awareness. It has been defined from a biological and causal perspective as the act of autonomously modulating attentional and computational effort, usually with the goal of obtaining, retaining, or maximizing specific parameters, such as food, a safe environment, family, or mates.
The issue of what consciousness is, and to what extent and in what sense it exists, is the subject of much research in philosophy of mind, psychology, neuroscience, cognitive science, and artificial intelligence. Issues of practical concern include how the presence of consciousness can be assessed in severely ill individuals; how non-human consciousness can be measured; at what point in fetal development consciousness begins; and whether computers can achieve conscious states."
Consciousness - Wikipedia, the free encyclopedia