Legally disowning a child under the age of 18 is known as child abandonment, and it is a crime that is punishable by law. When a parent disowns an adult child, it is also known as disownment.
The Free Dictionary defines disownment as rejecting the validity of a relationship or person. The Library of Economics and Liberty explains that disownment of an adult child is a serious decision that can be backed up by legal measures. The most common measure is to remove the adult child from the parent's will, but in extreme cases a no-contact or restraining order may be placed.