Conventional psychology defines "psychosis" as a symptomatic state of mental illness, a break in the patient's ability to function effectively. Psychosis is a catch-all term used by mainstream professionals to cover any number of sins and aberrations, and it has, by dint of the breadth of the field's reach, become nearly meaningless in and of itself. It simply labels the patient as "non-functional" without saying as much.
But there are states of fragmentary psychosis, wherein patients are functional—able to move about society without much trouble, able to care for themselves and make intelligent and moral decisions—but lack cohesion. The resilience of the human brain has self-corrected itself, building new networks around the blankness, thereby fragmenting off the area that has become "non-functional."
Invariably, this fragment rots, for lack of a better word. It begins to dissolve the barriers and the frameworks the patients have subconsciously erected. The slow rot begins to bleed into the patients' dreams and eventually begins to affect their waking consciousnesses.
If not properly treated, fragmentary psychosis can poison the entire system, resulting in a full psychotic breakdown. Unfortunately, since the accredited psychiatric industry doesn't recognize "fragmentary psychosis," these patients must suffer the dementia of the slow rot until they can manifest more recognizably psychotic behavior.
The oneirologist watches for the breaks, you know; the field of study concerns itself with identifying both the stressors and the fault lines that precede the patients' fragmentation. It can be such a little thing, the part of the mind that cracks, but it is always the catalyst that initiates a progression towards a larger systemic dissolution.