noun, noun which may take the genitive case particle 'no'
oldest member; longest-serving member
Refers to the person who has been in a group, organization, or team the longest. Often used in workplace or military contexts.
彼はこの部署の最古参だ。
He is the longest-serving member of this department.
最古参の社員として、会社の歴史をよく知っている。
As the oldest member of the company, he knows its history well.
最年長 refers to the oldest in age, whereas 最古参 refers to the longest tenure or membership.
Compound of 最 (most) + 古参 (seniority, long service).