Ego

Ego (the Latin word for “I”) is the unifying center of consciousness, the irreducible core of self-awareness—that which generates and sustains a sense of self, of personal identity. Our ego is not our thoughts, but that which thinks; not our judgments, but that which judges; not our feelings, but that which recognizes feelings; the ultimate witness

When your thoughts become your identity that is called ego. Ego is a case of mistaken identity. You believe that the illusory thoughts that come and go are your true self. Your true self is the dimension of you that does not come and go.

The Ego is that part of the psyche that believes in material existence.