The person who is at the moment in line to inherit
something that is passed down by the blood
line, such as a lordship
, but who is not guaranteed to succeed
, because some new person might be born with a closer claim
For example, if a lord has a daughter, but no son, that daughter may be heir-presumptive. If her father dies right now she would be his heir. But, in theory at least, he could have a son who would dispossess her. The son being male, even if born later, would take precedence in the order of succession, displacing the one who was previously the heir-presumptive. Likewise if the present heir-presumptive was a nephew or cousin of someone who did not (yet) have any children.
This is in contrast to their heir apparent, usually the eldest son, who cannot ever be displaced by someone else's birth.