Ego itself is a fiction created in the mind by circular patterns of thinking based on separation. So, 'the ego is the fiction in the mind' - what does that mean? [It means] that ego is basically our sense of self, and the thoughts, ideas and beliefs that circle around that sense of self that go into deriving a bigger, more conceptualized version of ourself. In other words: who we think and imagine ourselves to be.

The Basic Teachings - Part 1: Principles of the Teaching (2009)