You can delete most domains when they are no longer useful.
You can delete other types of domains, but not if the domain is assigned to an attribute. To delete the domain, you must first disassociate the attribute.
Deleting a domain does not affect values that have already been inserted on records. For example, if a user inserts a value in a field using a domain select value list, that value remains on the record. The record remains on the record even if the domain is later disassociated from the attribute and deleted.