noun
A person who tends to take a hopeful view of things; often used in contrast with 悲観主義者 (pessimist).
彼はどんな状況でも前向きな楽天主義者だ。
He is an optimist who stays positive in any situation.
Compound of 楽天 (optimism, from 楽 'comfort' and 天 'heaven') + 主義 (principle, -ism) + 者 (person).