noun

1. Imagination; fancy.

2. An imaginary quantity.

3. The set of values, institutions, laws, and symbols common to a particular social group and the corresponding society through which people imagine their social whole.

adjective

1. Existing only in the imagination.

   “Santa Claus is imaginary.”

2. (of a number) Having no real part; that part of a complex number which is a multiple of the square root of -1.

Synonyms: all in one's head