The age of majority is the legal age of adulthood as declared in law. It is the moment when a person ceases to be considered a minor, and assumes legal control over their person, actions, and decisions, gaining rights denied to them prior to that moment, while also terminating the rights a parent has over them.
Most countries set the age of majority at 18, but some jurisdictions have a higher age and others lower. The word majority here refers to having greater years and being of full age as opposed to minority, the state of being a minor. The law in a given jurisdiction may not actually use the term "age of majority". The term refers to a collection of laws bestowing the status of adulthood.