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1.
A brain twin is your cerebral soulmate. A person who shares the same ideology as you on a variety of subjects from music, books, and movies, to politics, art and culture.

A brain twin may be a close friend or someone you've never met, but whose work and opinions express that of your own.

"I was just thinking that!" or "I was just about to say that!" are common phrases used between brain twins.
OMIGOD. Your favorite TV show is 30 Rock and you read the Huffpo? We are totally brain twins.
by citizencain February 20, 2011
 
2.
Two people who are extremely close friends to the point that they love the same things, can easily complete each others sentences or talk simultaneously, and always know what the other person is thinking or would say in a situation. They get along perfectly and hardly ever fight. Other friends are jealous of their closeness and they are thought of as inseparable - you never see one without the other. Usually brain twins are two girls or a girl and a gay boy.
John, the popular jock, had finally gotten the nerve to ask Anna to the dance. She was, as usual, walking with her friends Allie, Cameron, and Zeke. When John approached them, he was happy how excited Anna looked to have been asked by him. But Zeke, because they were brain twins, knew only by looking at her that she would much rather have gone with anyone else, though John, Allie, and Cameron had been fooled.
by Phinny Fanny Phoo September 22, 2007
 
3.
These are two people that upon first interaction have an erie mental connection. This includes but is not limited to constantly reading the others person's mind, finishing sentences, sharing unspoken thoughts, jinxing, and laughing upon eye contact because they are both thinking the same thing. A brain twin knows how the other brain twin feels about most all people and topics. This is because brain twins think exactly the same way. Brain twins get along famously and develop close relationships. Other people will often feel like left out when brain twins get together. People may have more than one brain twin although it is extremely rare. Side note: Although not determinative, sexual feelings/tension are likely to develop with co-ed brain twins, due to the egocentric nature of humans.
Brain Twins 1&2 immediately after watching a rousing international hockey match at a local pub-
BT1: Soo...
/BT2 reaches for keys

(Simultaneously)
BT1: Your place??
BT2: My place??

/Intense Rough Sex
by Brain Twin February 28, 2013