The incumbent is the current holder of an office or position. In an election, the incumbent is the person holding or acting in the position that is up for election, regardless of whether they are seeking re-election.
There may or may not be an incumbent on the ballot: the previous holder may have died, retired, resigned; they may not seek re-election, be barred from re-election due to term limits, or a new electoral division or position may have been created, at which point the office or position is regarded as vacant or open. In the United States, an election without an incumbent on the ballot is an open seat or open contest.