There are various synonyms for the term "union card," which is a card that proves membership in a union. One synonym for union card is "membership card," as it represents the person's membership in the union. Another synonym is "labor card," which refers to the individual's association with a labor organization. Some other synonyms for union card include "working card," "trade union card," and "guild card." These cards are essential for workers to be represented by a union and enjoy benefits such as collective bargaining agreements, workplace protections, and better wages and benefits.