Merriam-Webster lists the three basic meanings of the word.

At the most basic level, ego denotes that part of the perceiving and thinking entity which is not anything else. Little children seem not to be entirely aware of that distinction; it is at least in part a learnt thing.

You touch the second meaning, self-esteem, in your own question ("a fragile ego").

Freud, who I would not directly count as a philosopher, used the word as the "conscious" part of our psycho-physical being.

Merriam-Webster lists the three basic meanings of the word.

At the most basic level, ego denotes that part of the perceiving and thinking entity which is not anything else. Little children as well as many animals seem not to be entirely aware of that distinction; it is at least in part a learnt thing.¹

You touch the second meaning, self-esteem, in your own question ("a fragile ego").

Freud, who I would not directly count as a philosopher, used the word as the "conscious" part of our psycho-physical being.

¹ And if we think about it, in the beginning of a child's existence its limits are not well defined at all; even birth and cord-cutting, fundamental as they are, do not sever the connection to the mother completely. There is an ongoing bodily, emotional and, if you want, spiritual connection.