Definition: a realistic respect for a favorable impression of oneself. Usually it is stable, however it can change based on events, relationships, health and appearance. The fact that it goes up and down is a proof that self-esteem can change.
Self-esteem is the healthy stance of a continuum alternating between self-defeating shame and self-defeating pride.
Self-defeating pride represents huge unrealistic ego, arrogance and looking down on others. Insecurity is the cause of it due to lack of good healthy parenting.
Self-defeating shame is people thinking they are less than the standards around them. Also in an unrealistic, unappreciative way of who they really are.
Self-esteem on the other hand is a realistic view of oneself based on being human with limitations and faults but also with gifts and strengths. They like themselves despite their weaknesses and know how to navigate their lives around them.
There are secondary gains to staying with low self-esteem as well as negative consequences.
Building self-esteem requires love, support, unconditional belief of self-worth which will enhance growing and maturity physically and emotionally which ultimately will lead to a healthy realistic appreciative view of oneself.