There are a lot of words you hear associated with plurality, in unusual ways. Other people on the internet may disagree with our usage of these terms, and that is okay. This is just how we use them.
Plurality / Multiplicity – a situation where there’s a body through whom multiple people may be perceived.
Singlet – someone who is not part of a plural group; a single person in a single body.
System – this used to be a neutral term referring to a body that exhibits plurality. Or more precisely, to the group that resides in it. However, we do not recommend using this word anymore, because it has been claimed by a loud subculture of medical determinists.
Gateway System – a specific kind of system where the people who front are residents of an inner-world/other-world. Same caveat on “system” here, but really, this has been the term for decades.
Headmate / Systemmate – A member of a plural system. Please don’t use the words “alter” or “personality” unless someone explicitly tells you to do so. Headmate can imply false things as well. We are fond of simply using the term person. It’s not too hard to figure a way to phrase it.
Inner-world / Other-world / Headspace – A place that members of a system may reside and/or visit. “Inner world” and “headspace” are more common in the larger community, but we like “other world” because it doesn’t have any implication about where it’s located. Gateways often just happen to be connected to that other place, and it exists outside of them. (Keeping in mind that this is their subjective experience, whether you believe in it or not.)
Front – Control over the shared body. This act is called “fronting“. Likewise, you may have “co-fronting“, where there is more than one.
& – This isn’t exactly vocabulary, but it is becoming common. It is a way to form a plural from something that is not usually explicitly plural, like “you”. “How are you&?” implies asking how everyone is doing. Likewise, signing a message “- Fuzzy&” might imply, “Fuzzy and others”.
DID/MPD – These are psychiatric diagnoses for someone who is plural, usually by way of trauma. Many visited a doctor/therapist because of feeling disordered or dyscommunicative, but not all of them are.
It is a common misconception that all plural systems fall under this category, but many do not. In general, being a disordered system with broken communication and responsibilities is something that needs help/therapy, not simply being plural. It is the same as with any group of people who must live together. (There are other reasons for plural groups to need help, but I am not going to dive deep on that topic here.) But imagine how ridiculous it would sound if someone suggested that anyone who lives with someone else needs help in a major way.