Imaginary friends or imaginary companions are a psychological and social phenomenon where a friendship or other interpersonal relationship takes place in the imagination rather than external physical reality. Although they may seem very real to their creators, children usually understand that their imaginary friends are not real. The first studies focusing on imaginary friends are believed to have been conducted during the 1890s.
Imaginary friends are made often in childhood, sometimes in adolescence, and rarely in adulthood. They are people or other characters created in the mind that the creator interacts with. In some studies, imaginary companions are defined as children impersonating a specific character (imagined by them), or objects or toys that are personified.  However, some psychologists will define an imaginary friend only as a separate created character. Imaginary friends or imaginary companions can be people, but they can also take the shape of other characters, such as animals, or other abstract ideas such as ghosts, monsters, angels, etc.  These characters can be created at any point during a lifetime, though Western culture suggests they are most acceptable in preschool and school age children.  They often function as tutelaries when played with by a child. They reveal, according to several theories of psychology, a child's anxieties, fears, goals and perceptions of the world through that child's conversations. They are, according to some children, physically indistinguishable from real people, while others say they see their imaginary friends only in their heads, and still others cannot see the friend at all but can sense his/her presence. Once children reach school age, boys and girls are equally likely to have an imaginary companion.  And research has often reiterated that there is not a specific "type" of child that creates an imaginary friend. 
Research has shown that imaginary companions are a normative part of childhood, and even adulthood.  And some psychologists  suggest that imaginary companions are much like a fictional character created by an author. As Eileen Kennedy-Moore points out, "Adult fiction writers often talk about their characters taking on a life of their own, which may be an analogous process to children’s invisible friends." In addition, Marjorie Taylor and colleagues have found that fiction writers are more likely than average to have had imaginary companions as children.
There is a difference between the common imaginary companion that many children create, and the imaginary companions of psychopathology. Often when there is a psychological disorder, and an imaginary companion is present, the creator believes that this companion is real, and does not differentiate between the real and imagined. 
It has been theorized that children with imaginary companions may develop language skills and retain knowledge faster than children without them, which may be because these children get more linguistic practice than their peers as a result of carrying out "conversations" with their imaginary friends.
Kutner (n.d.) reported that 65% of seven-year old children report they have had an imaginary companion at some point in their lives. He further reported:
Imaginary companions are an integral part of many children's lives. They provide comfort in times of stress, companionship when they're lonely, someone to boss around when they feel powerless, and someone to blame for the broken lamp in the living room. Most important, an imaginary companion is a tool young children use to help them make sense of the adult world.
Taylor, Carlson & Gerow (c2001: p. 190) hold that:
despite some results suggesting that children with imaginary companions might be superior in intelligence, it is not true that all intelligent children create them.
Some psychologists[who?] have suggested that older children may retain but stop speaking about imaginary friends due to adult expectations and peer pressure. Pediatrician Benjamin Spock believed that imaginary friends past age four indicated that something was "lacking" in the child or his environment. Some have theorized that children who hold on to imaginary friends past school-age are stigmatized.
Other professionals feel that imaginary friends are common among school-age children and are part of normal social-cognitive development.Part of the reason that people believed that children gave up imaginary companions earlier than they do, is related to Piaget's stages of cognitive development. Piaget suggested that imaginary companions disappeared once children entered the concrete operational stage of development.  Marjorie Taylor identified middle school children with imaginary friends and followed up six years later as they were completing high school. At follow-up, those who had imaginary friends in middle school displayed better coping strategies but a "low social preference for peers." She suggested that imaginary friends may directly benefit children's resiliency and positive adjustment. Because imagination play with a character involves the child often imagining how another person (or character) would act, research has been done to determine if having an imaginary companion has a positive effect on theory of mind development. 
- Wilson, imaginary friend of Chuck Nolland in Cast Away.
- Tony, imaginary friend of Danny in The Shining.
- Charles Herman, imaginary friend of John Nash in A Beautiful Mind.
- Captain Howdy, imaginary friend of Regan McNeil in The Exorcist
- Ivan, imaginary friend of Trevor Reznik in The Machinist.
- Fred, imaginary friend of Elizabeth Cronin in Drop Dead Fred.
- Tyler Durden, imaginary friend of the unnamed narrator in Fight Club.
- Harvey, imaginary friend of Elwood P. Dowd in Harvey.
- Larry Houdini, imaginary friend of Darwin, and Zoe, imaginary friend of Frances in Don't Look Under the Bed.
- Marc Imran and Elle Maya, imaginary friends of Hareez in Real Life.
- Frank, imaginary friend of Donnie Darko.
In other media
- Bloo, imaginary friend of Mac, and many of the characters from Foster's Home for Imaginary Friends.
- Plank, imaginary friend of Jonny 2x4 in Ed, Edd n Eddy.
- Gretchen, imaginary friend of Elizabeth (Stephanie Berrington) in Father Goose: His Book.
- Nadine, imaginary friend of D.W. in Arthur.
- Millie the Mermaid, imaginary friend of Zuri in Jessie.
- Sean Murphy, imaginary friend of Martin Moone in Moone Boy
- Soren Lorensen, imaginary friend of Lola in Charlie and Lola.
- Squirrly (a deceased pet squirrel), Jeffy (a garden hose), Vanessa (an apple), Dewey (a paper towel roll), and Boxy Brown (a cardboard box) are imaginary friends of Meatwad in Aqua Teen Hunger Force.
- In The 7D episode "Buckets," the ghost girl Buckets (played by Kate Micucci) turns out to be the imaginary friend of Queen Delightful (who was nicknamed "Mopsy" by Buckets).
- Taylor, M. (1999) Imaginary Companions and the Children Who Create Them. New York: Oxford University Press.
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- Taylor, Marjorie; Carlson, Stephanie; Maring, Bayta; Gerow, Lynn; Charley, Carolyn. "The characteristics and correlates of fantasy in school-age children: Imaginary companions, impersonation, and social understanding.". Developmental Psychology 40 (6): 1173–1187. doi:10.1037/0012-16188.8.131.523.
- Gleason, Tracy (2013). The Oxford handbook of the development of imagination. Oxford: Oxford University Press. ISBN 978-0-19-539576-1.
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- Gopnik, Alison (2010). The philosophical baby : what children's minds tell us about truth, love, and the meaning of life (1st Picador ed. ed.). New York: Picador/Farrar, Straus and Giroux. ISBN 978-0-312-42984-3.
- Kennedy-Moore, Eileen (2013) "Imaginary Friends: Are invisible friends a sign of social problems?" Psychology Today; Growing Friendships blog. 31 January 2013.  (accessed: 24 May 2013)
- Taylor, M., Hodges, S. D., & Kohányi, A. (2002-2003). The Illusion of Independent Agency: Do adult fiction writers experience their characters as having minds of their own? Imagination, Cognition and Personality, 22(4), 361-380.
- University of Manchester (8 March 2005). "Imaginary Friendships Could Boost Child Development". ScienceDaily. Retrieved 9 November 2011.
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- Taylor, M., Hulette, A.C., Dishion, T. J., (2010). Longitudinal Outcomes of Young High-Risk Adolescents With Imaginary Companions. Developmental Psychology, 46(6), 1632-1636.
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- Imaginary Friends with Dr Evan Kidd podcast interview with Dr Evan Kidd of La Trobe University