Another excellent question. So what exactly is a lucid dream? Various definitions of the experience have emerged in the literature. The simplest of these states that lucid dreams are those in which the subject is aware that he or she is dreaming. Other researchers have added a qualifier: that one has to become perfectly or fully aware that one is dreaming. Exactly what is meant by the terms "perfectly" or "fully" is usually not explicitly stated, but usually involves the ability to consciously exert control over events in the dream scenery. It should be noted, however, that even though lucidity in dreams is often accompanied by varying degrees of dream control, this ability is not in itself a sufficient indicator of lucidity. A broader and more precise definition of what constitutes a lucid dream is given by Stephen LaBerge who suggests that the consciousness experienced by a lucid dreamer is not unlike that which is experienced during the waking state. Thus LaBerge writes that "the lucid dreamer can reason clearly, remember freely, and act volitionally upon reflection, all while continuing to dream vividly.” Similarly, Tart (1979) states that a lucid dream consists of more than just having the dreamer realize "This is a dream." Like LaBerge, he suggests that in a lucid dream "the 'higher' mental processes that we think of as characterizing waking consciousness, such as memorial continuity, reasoning ability, volitional control of cognitive processes, and volitional control of body actions (at least for the dream body), all seem to be functioning at a lucid, waking level.” Others have also adhered to this conceptualization of the lucid dream state.
It would appear then that the lucid dream experience may be best understood if placed on a continuum. At one end we would have what may be called low-level lucidity, in which an individual may realize that he or she is dreaming, but then wake up, or simply relapse into non-lucid dreaming. In the middle of the continuum would fall those lucid dreams in which the dreamer, in addition to knowing that he or she is dreaming, can also exert some degree of control over the dream environment and retain some but not all of his or her waking mental faculties. Thus a person in this situation may be able to move about in the dream scenery as he or she pleases, but may be unable to alter some aspects of the dream, remember what day it is, or remember what their agenda for the following day consists of. At the high end of the continuum are those dreams in which an individual can exert a considerable amount of control over the dream content and, most importantly, is in possession of his or her mental faculties to the same extent as if the person were fully awake.
To this continuum should also be added what Celia Green has termed "pre-lucid dreams" as well as the phenomenon of "false-awakenings." The former refers to those dreams "in which the subject adopts a critical attitude towards what he is experiencing, even to the point of asking himself 'Am I dreaming?' but without realizing that he is in fact doing so.” The latter refers to those dream experiences in which one dreams that one has woken up, usually in their normal sleep environment. Both of these phenomena are known to occur in lucid dreamers, especially novices.