Other forms: unionized; unionizing
To unionize is to form a group of workers that has more power to negotiate with an employer. Fast food employees might unionize so they can bargain for higher wages and safer working conditions.
When workers unionize, they vote to join a union, or a collective bargaining group. Sometimes belonging to a union is easy: most public school teachers, police officers, and other municipal employees automatically join existing unions when they're hired. In non-union jobs, however, like waiting tables or delivering food, employees who want to unionize have to convince a majority of their coworkers to vote in favor of the move.