a.

1. 想像中的；虚构的；幻想的
   an imaginary friend

   想像中的朋友

   All the characters in this book are imaginary.

   这本书中的人物全是虚构的。

2. 【数】虚数的

adj.

1. existing only in the imagination.
2. Mathematics  (of a number or quantity) expressed in terms of the square root of -1 (represented by i or j).