A person's sense of their own worth and ability.
Ego can be good or bad. It can make people feel strong and important, but it can also make them act like they are better than others.
Examples:
His ego is very big.
She has a big ego.
My ego is not as big as his.
His ego is so big, it needs its own room!
Her ego is so big, it can't fit in the house!
My ego is as big as a mountain.