Imaginary friends.
Meaning: This refers to people or characters that exist only in the mind or imagination of a person. It's like saying, Characters or people that you create in your mind and pretend are real.
Choice of Words: Imaginary is an adjective that means existing only in the mind or imagination. Friends is a plural noun that refers to people whom one knows and with whom one has a bond of mutual affection. When you put them together, "Imaginary friends" is a phrase that describes people or characters that exist only in the mind or imagination of a person.
Alternative Expressions
Related Expressions