What is "a" multiple?
A multiple is also someone with (what the doctors persist in calling) multiple personalities or (nowadays, and only in the United States) Dissociative Identity Disorder.
The former is not quite accurate, hence the rise of the simpler term "multiple." That is, the various members of a multiple system are not one person's many "personalities," but individual complete people.
According to Dr. Ralph Allison (who specializes in issues specific to multiples) the latter clinical term was changed from Multiple Personality Disorder because many doctors did not believe in multiplicity and felt that the term "multiple personality disorder" lent it too much credence. "DID," therefore, was selected to better suggest that such patients' main disorders consisted of thinking that they were many instead of one.
Thus, the "cure" devised for multiples and MPD/DID was to "integrate all that person's parts," or to somehow make all the people in a system into one - a practice which is barely understood (or understandable) and often fails. Interestingly, this term (DID) is used almost entirely within the United States; the rest of the world relies mostly on the ICD, which still calls it MPD.
Why all the "airquotes"?
Most of the words above are terms that come from therapists and psychologists. They are the wacky little words that they use to describe us even when they don't correspond to our experiences. (For example, many people in multiple systems hate being called parts, aspects, personalities, and/or alters. My ex-therapist couldn't stop doing it for the life of her.)
I came up with a Handy Multiple Glossary for a workshop once:
Alter, Personality - Terms for people in a multiple system. These terms are often considered derogatory or inaccurate because they stem from an implication that there is one "real" person who has lots of "personalities." Try "person" instead.
Being Out - Both a term for not hiding who you are, and a term meaning that someone is the person or people who are currently "out in the body" doing regular outside stuff like getting ready for bed or talking to friends.
Co-conscious - A state where two or more people are out at the same time, each aware to some extent of what the other is thinking. Alternatively, a state where different people in the system have some access to each others' memories, so they can function consistently in the outside world.
DSM-IV - Stands for Diagnostic and Statistical Manual, 4th edition. It is within this manual that the diagnosis of Dissociative Identity Disorder can be found.
Fronter - Someone who spends a lot of time out, taking responsibility for the body and the outside life. There are often many fronters in one system. Sometimes they are called "hosts," but that can also be a derogatory term if it's used to imply that the "host" is the only "real" or valuable person in the system.
Integration - Presented by many therapists as the "cure" for multiplicity, where the ultimate goal is to make everyone merge into one person somehow. Multiplicity has been very little understood and explored so far, partly because of the reliance of therapists on integration. This in turn means that integration often does not work, resulting in the deaths of people within a system or in a re-emergence of the supposedly integrated members. In other cases, integration just ends up meaning that everyone becomes co-conscious. It is a controversial act and issue in the multiple community. Ultimately, it should only be up to the people within a particular system whether to integrate or stay separate, and to figure out what that means for them.
Losing Time - The experience of being inside while someone else was out, and missing the chunk of time that they were there. Losing time can also occur when a person in the system is out, but cannot retain memories of events, as with dissociation and repressed memories.
Mid-Continuum - Mid-continuum is to multiple as genderqueer is to transgendered: completely different but totally the same and not yet fully explored even by people within its community. Also known as identityqueer. (I know, it's confusing. About all I can say is that this is often characterized by people saying "I don't think I'm multiple, but it's like there are a lot of people in here, but they're all me." However, co-consciousness can also make everyone in a system feel like "me" to whoever is out.)
Switching - When someone else in the system comes out to replace whoever was there before (as opposed to simply being co-conscious with them). This can be voluntary, but often is not.
System - A group of people sharing the same body. Many systems come up with a collective name for themselves, like DarkSouls or Kaleidoscope House, so that people can refer to the group more easily. (For example, if I was hanging out with a multiple friend and wasn't sure who was out at a given moment, I might say, "Oh, the Chimera were telling me...." Whereas if they didn't have a system name, I would have to find some other way of indicating that I didn't know who was out, like "Oh, Jane, I mean, or someone in there anyway, was telling me....")
How do I know whether I'm multiple?
It's different for everyone. I figured it out for myself because I was dating someone who was multiple, who invited me to join this multiple listserv... and I did, and realized that I identified with way too much of the experiences that people described there.
Eventually, I put together a little poll which listed some of them:
Do you do things you can't explain?
Do you own stuff that you don't remember buying?
Does your outlook on life, or do your goals or political opinions, change completely from day to day or week to week?
Do you have a hard time remembering what different times in your life were like - such as remembering what it felt like to be in high school after you graduate - even if the change was just earlier that year?
Do you experience something similar when you go to work, or into a store - a decreased awareness of wherever you had been before or were going next? Such that where you are feels almost like the only real place?
Do you have to pack extra books or clothes when you go on a trip because you have are so concerned and confused about what you'll feel like wearing, reading, or doing when you're there?
Do you ever totally forget what you said not even a second ago - like you didn't even hear it?
Do you frequently have mental conversations with yourself? Like you're your own peanut gallery?
Do you ever question (or have you in the past) whether you might be multiple?
Do people ever mention things that you've said or done that you don't remember, or reference conversations you don't recall having with them, even though they were apparently recent?
Does the rate at which time passes seem completely confusing to you?
Do you ever get that thing, where, like, you'll be having a conversation or staring off into space or whatever, and there'll be like this click, but it's not an audible click noise - it's like a mental click-into-place - and all of a sudden you're like, "Oh, now I'm paying attention," -- like the scene starts anew for you - it doesn't rewind or anything, it's just like now you're there? But you were there before and you were paying attention before, only now it feels different?
If someone asks you what you did yesterday, or last weekend, or this afternoon, do you find that your mind is often a blank at first - if you can even access that information?
They were ranked, originally. The "clicking question," for example, nets the test-taker more points because it's as clear a description of switching as we've ever found. Others are worth only one point, because things like changing one's political opinions frequently can be explained other ways.
The point, though, in that and other examples, is not that someone is changing their opinion, but that their opinion seems to change without any extra thought or input.
As with any issue of identity, the easiest and best way to really figure out whether you belong under this umbrella term is to go get to know people in this community and understand what they're experiencing.