But this is not a problem, since you don't need one.
If I ask you "Are you alive?" then your answer must be "Yes" even though coming up with an English-language definition of "alive" is a daunting task.
The plain fact is that we all know damn good and well that we're conscious when we're logged onto the JREF forum and posting on threads.
To pretend otherwise is silly, at best.
I don't know, when I was doing crisis work and had a variable schedule and sleep disturbance, there were some times I am not sure I met the criteria fully. Now in some senses I did (certainly more so than twilight anesthesia or sleepwalking.)
If we go with the American Heritage Dictionary:
con•scious (knshs)
ADJECTIVE:
1. Having an awareness of one's environment and one's own existence, sensations, and thoughts. See Synonyms at aware.
(5%-15%)(There were times my awareness was at a very low level and not attentive, yet I posted.)
2. Mentally perceptive or alert; awake: The patient remained fully conscious after the local anesthetic was administered.
(5%-15%)(There were times I was barely alert and only sort of aware, sleep disturbance is strange, especially when I was not treating my sleep apnea. Yet I posted.)
3. Capable of thought, will, or perception: the development of conscious life on the planet.
(100%)(Applicable, I was capable of thought, not always coherent or lucid.)
4. Subjectively known or felt: conscious remorse.
(5%)(This is the area where is gets dicey, there were times I was posting in a sort of fugue, sort of known, sort of not.)
5. Intentionally conceived or done; deliberate: a conscious insult; made a conscious effort to speak more clearly.
(5%)(Mixed bag, some intent some not.)
6. Inwardly attentive or sensible; mindful: was increasingly conscious of being watched.
(5%)(Not too attentive, depends on the meaning of sensible)
7.Especially aware of or preoccupied with. Often used in combination: a cost-conscious approach to further development; a health-conscious diet.
(Doesn't apply)
Now I have chosen a time period of my life where there was a perfect storm of factors, depression and OCD, rotating cycles of intermittent sleep and waking (on call one week out of four 24/7), large amounts of caffeine, disturbed sleep (other than waking at odd hours), sleep apnea and a dose of Zoloft at 150mg daily. This led first to the chaotic sleep wake cycle especially that one week out of four, but the sleep apnea and depression led to severe sleep disturbance along side the strange sleep cycle. There are times I would dream intensely as soon as I went to sleep, wake up , dream some more, sleep deeply, and then startle awake for an extended period. Part of this was the stimulation of the caffeine and Zoloft as well, and the stress related symptom of insomnia. (Fortunately most of this has resolved by treating the sleep apnea, I now maintain on a lower dose of Zoloft, have a regular sleep wake cycle and have only one cup of coffee/day.)
Now I have chosen common criteria (my preferred dictionary) but also an extreme time in my life where I was often functional but only marginally awake and alert. I do believe there are times I posted when I was essentially sleep typing.
More strange, I would take crisis calls in the middle of the night, there were two times that I recall where I was barely awake and became more aware as the call progressed, now these were skills that had been honed and practiced. But there were some strange moments where it was sort of avolitional.