noun
retired person (at retirement age); retiree
Specifically refers to someone who has retired upon reaching the company's mandatory retirement age, not just any retired person.
父は定年退職者で、今は趣味の園芸を楽しんでいる。
My father is a retiree who reached the mandatory retirement age, and now he enjoys gardening as a hobby.
会社は定年退職者を再雇用する制度を導入した。
The company introduced a system to rehire employees who have reached the mandatory retirement age.
退職者 is a broader term for any retired person, regardless of whether they retired at the mandatory age or voluntarily. 定年退職者 specifically means retirement at the mandatory age.
Compound of 定年 (mandatory retirement age) + 退職 (retirement) + 者 (person).