Multiple personality, or multiple personality disorders (MPD) is when a person is displaying two or more seperate personalities.
These personalities often go by different names and the stronger new/stronger personality is often aware of the orignal/others whereas the original personality often just recalls 'blacking out'.

Psychologists Thigpen and Cleckley conducted a study on a woman claiming to have 'blinding headaches', which concluded she infact was one of the vary rare cases of reported MPD victims. (For further reading, this study can be found here: http://www.holah.co.uk/, and is explained well in the film based on this study 'The Three Faces Of Eve').
(Taken from Thigpen and Cleckley's study on multiple personalities)

During the interview, Eve White who was normally very self-controlled became distressed and asked whether hearing an occasional imaginary voice made her insane.

She reported that she had on several occasions over the last few months briefly heard a voice addressing her. During this conversation Eve White, as if in pain suddenly put both hands to her head. After a tense moment of silence her hands dropped, and the therapist observed a ‘quick, reckless smile’ and in a bright voice she said: ‘Hi there, Doc’
by Alice Batchelor May 11, 2007
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A mental disorder that is now called Dissociative Identity Disorder. This disorder is caused by trauma. Someone with this disorder will have at least 2 separate personalities that take control of the body at different times.
by Thelabrinth May 10, 2015
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1. Multiple Personality Order is a common response to the largely conflicting views and ambitions pursued by different groups in the world, related to artistic "disinterestedness".

A condition in which the intelligent individual decides to order his or her mind by creating different personas to better associate with the various Single Personality people in the world.

It is often easier to accomplish by giving the different personalities their own ridiculous names to help in the mental transition.

2. A social order, or club, in which the members have multiple personalities, unlike most orders or clubs where everyone dresses the same way, thinks the same way, says the same boring nonsense and even eats the same snacks when they have their coffee breaks.

These are quite rare.
1.

Student: "Hi Class, my name is David Cock, today I will be giving you a presentation on the effective use of arbitrage in the venuzuelan oil industry and the successful use of grassroots movements to overthrow tree-hugging presidents."

Harvard Business Professor: "But you're registered as Agnes Gonxhe Bojaxhiu..."

Student: "I don't know what granola-eating nonsense is spewing from your underfed academic book-reading face, I have to pass your class on corporate sabotage or my dad will beat me, and nothing will stop me."

Classmate: " Oh there goes mother teresa again with her multiple personality order..."
by kidneyswap September 23, 2010
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when a person talks to, and frequently answers themselves, possibly acting as if they are having a conversation with multiple people... with themselves
(to yourself)
person 1-hi
person 2-fuck off
person 3-thats not nice
person2-i dont care
3 u should
1 stop talking to him hes mean
4 HI
5 what are you, retarded
3 thats my best friend dumbass
2 i still want you fuck off
9 hey guys, watsup
1,2,3,4,and 5 ....

multiple personality disorder
by BOOM TITTIES October 3, 2011
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When a person involved in a business organization holds multiple roles and genuinely begins to think that they themselves are different people by virtue of the roles. Often happens when a shareholder is a director who is also a director of an entity that is a shareholder. People with this syndrome can be caught emailing themselves with the to/from fields being populated by the different roles they hold. Victims of this syndrome often cite bizarre legal theories to substantiate spending time emailing themselves in official capacities.
from: johnasmith@companymail.com
to: johnasmith@personalmail.com

Dear John A Smith, Shareholder:

Enclosed please find our latest annual report.

With Best Regards,

John A Smith (the same guy as the recipient)
CEO

Secretary or other clerk reading this exchange: "This guy has Corporate Multiple Personality Syndrome."
by Misanthropic Views October 9, 2013
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(Abbreviated to FMPD) Someone who portrays themselves on Facebook completely differently to how they actually are/behave in real life.

i.e A quiet introvert in real life, but who frequently comments, makes jokes/banter over Facebook. Polar opposite personalities
"That Tommy has got serious Facebook Multiple Personality Disorder! Quiet as a mouse in real life... yet on FB he's the life and soul of the party..."
by BobajobRob February 7, 2012
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Online disorder in which a person posting comments, blogging, chatting, or otherwise electronically communicating, especially while under anonymous screen names, uses multiple screen names to enhance different parts of their offline multiple personalities, and becomes someone and/or something they are not under each name.
(Not to be confused with Online Personality Disorder (OMPD)
"Gladys has 7 AOL Accounts and 49 different screen names she types under- A different one per week. That one is a doctor, that one is a lawyer, that one is a housewife...she uses that one to talk to these people...that one to talk to those people..."
She has Online Multiple Personality Disorder (OMPD)
by JayyTheOrc January 4, 2009
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