The term 'One Who's Next in Line' can be summarized as an HEIR, which is a four letter word. An HEIR is someone who is legally entitled to receive the property of another person upon their death. This could be a family member, such as a child or spouse, or it could be someone designated by the deceased in a will. In some cases, an HEIR may also be someone who has been appointed to take over the responsibilities of another person, such as a business partner or a political leader.