Here is a cut and paste from a discussion I read on what the terms mean:
After checking with a couple of doctors, one on each coast to account for any variation in local definitions, I can state with some certainty that most of these terms don't mean a whole heck of a lot and vary depending on the hospital and the doctor's definition.
"Critical" does seem to mean something fairly standard. As one doc told me, critical condition, by definition, requires care in a critical care or intensive care unit. In general, it's the worst condition (other than, obviously, DOA) a patient can be in, with a high risk of death within the next 24 hours.
Being upgraded to "serious" means there is a lower likelihood of death within 24 hours, but the patient still requires close observation. So, you're definitely thought to be better off in "serious" condition than in "critical" condition, but you're not ready to go run a marathon, or for that matter shuffle to the bathroom.
Another term you've probably heard is "stable," as in, "the patient is in serious but stable condition." This means there are no major active medical issues and the patient's basic vital functions are not fluctuating or in need of support. So they are seriously injured, but not expected to die from those injuries.
There are a host of other terms (you may have heard "good" and "satisfactory" as well, for example), but these are more or less judgment calls without clear definitions. They're basically an official-sounding way to say, "don't worry, he or she will be fine."
Terms like critical, stable, serious, etc., are mostly for the benefit of the media. Most doctors don't use these words when talking with the families of their patients.
