noun
full-time worker dedicated to one role; full-time union official
Often refers to a person who works exclusively for a labor union, but can also mean any worker assigned to a single specialized task. The union sense is especially common in Japanese labor contexts.
彼は組合の専従者として活動している。
He works as a full-time union official.
その会社では、専従者が品質管理だけを担当して
At that company, a dedicated worker handles only quality control.
専任 means 'full-time assignment' or 'exclusive responsibility' and is more general; 専従者 often implies a labor union context.
Compound of 専 (exclusively) + 従 (serve, follow) + 者 (person). The term became common in post-war labor movements to describe union staff working full-time for the union.