noun
altruist
A person who practices altruism; a somewhat formal or philosophical term.
彼は真の愛他主義者として知られている。
He is known as a true altruist.
Synonym; 利他主義者 is more common and uses the standard kanji 利他 (benefit others). 愛他主義者 is less frequent and may carry a slightly more emotional nuance of 'love for others'.
Compound of 愛他 (love others) + 主義 (principle) + 者 (person). The term is a direct translation of the English 'altruist', likely coined in the Meiji or Taisho period as part of philosophical vocabulary.