A more thorough explanation:

Definition: A stepmother is the wife of one's father by a later marriage.

For example, if a man gets married for the second time and his new wife has no biological relation to his children from his previous marriage, she becomes their stepmother.

This definition is important because it clarifies the relationship between a stepmother and her stepchildren. It also distinguishes her from other types of mothers, such as biological mothers or adoptive mothers.