unsuccessful candidate; defeated candidate; losing candidate
Refers to a person who failed to be elected or selected in an election, contest, or selection process.
He became an unsuccessful candidate in the election.
Among the defeated candidates, some will try again in the next election.
落選 is the noun/verb 'election loss' or 'being rejected in a selection', while 落選者 specifically refers to the person who lost.
Compound of 落選 (election loss, rejection) + 者 (person).