Nilauto
Before I talk about this, I want to make it clear that this was heavily, heavily based on the term autnil, which I didn't make and don't intend to speak over. I wanted to make something similar for someone in my system, but with some differences and general broadness because autnil didn't fit quite right. I made this for me and my system, and that's about it. If you want to use it, you're welcome to, but please don't forget to credit the person who made autnil too if you put this anywhere.
Nilauto is a term we use to describe a lack of an internal "self," or a complete inability to perceive it. In simpler terms, if being a plural (system) is multiple, and being a singlet is one, then being nilauto is zero. It can also be used to describe a spectrum of experience that is in the "opposite" direction from plurality, going from less than one as opposed to more than one.

It can be a state that someone simply is in, with no known "cause" for it, or it might be caused by a variety of psychological and / or psychiatric experiences such as dissociation, self-disorders, and I'm sure much more than that. In other words, it can be "disordered" or "non-disordered." It might be something you've always been, or it could be something you've developed or become. There is no inherent prerequisite to using the term nilauto other than experiencing nilauto...ness? Nilautonomy? Nilautomy? As an experience in some way.
While the term absolutely covers a variety of medical experiences, it in itself is obviously not a medical term. Instead, it's meant to be something that describes someone's self in a broad sense, grouping together a variety of experiences under a term that can make it easier to explain / pinpoint, as well as find people with shared experiences.
Some nilauto things (you don't have to be the same as all of this to be nilauto, this is just a personal list of examples)
- Feeling as though your own internal thoughts do not belong to anyone, as though it is disembodied and disconnected from yourself; or, not perceiving internal thoughts at all.
- Feeling as though underneath your "shell," there is nothing at all, or something approaching nothing.
- A strong disconnect from the sense / idea of having a "self."
- Only identifying with your material self- ex: "I am a body with some nerves and organs," and nothing else.
- Lacking an identity, or feeling as though you do not want or need one.
- Being unable to perceive yourself internally, or think of "yourself" in a tangible way.
The term "nilauto" comes from nil (zero) and auto (self); ie, zero self. It is pronounced "n-ihl-awh-t-oh," or in compared words, "n" + "ill" (as in the term for when you're sick) + "auto"(said the same way as you would in automobile or automatic.)
Before someone asks: yes, system members can be nilauto, or at least on the spectrum of it. It might sound contradictory at first, but I don't think it is. Nilauto is about your internal self, so even if someone is within a plural system, they might experience these things as individuals in a wide variety of ways.
I made a personal flag for the term, which, like the term itself, is heavily based on the autnil one, just with a different layout and a slightly different symbol:
