noun
unsuccessful candidate; defeated candidate; losing candidate
Refers to a person who failed to be elected or selected in an election, contest, or selection process.
彼は選挙で落選者となった。
He became an unsuccessful candidate in the election.
落選者の中には、次回の選挙に再挑戦する人もいる。
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).