An egomaniac is someone who thinks they are hugely important, and does not care if they upset other people in order to get what they want. The word is informal and sharp: we reach for it about a boss, a star, or anyone who seems to live only for their own name and power. It is an insult, not a calm description. Though actually, the 'maniac' piece here is the everyday English pattern — someone wild about one thing — not a doctor’s word. You are not naming an illness. You are saying this person’s self has grown so large that other people become furniture. You would not say it about a friend unless you meant to sting. Egotist is milder: an egotist talks about themselves. An egomaniac will push you aside.