A trade union (labor union in American English), often simply referred to as a union, is an organization of workers who have come together to achieve common goals, such as protecting the integrity of their trade, improving safety standards, and attaining better wages, benefits (such as vacation, health care, and retirement), and working conditions through the increased bargaining power wielded by solidarity among workers. Trade unions typically fund the formal organization, head office, and legal team functions of the trade union through regular fees or union dues. The delegate staff of the trade union representation in the workforce are made up of workplace volunteers who are appointed by members in democratic elections.

The trade union, through an elected leadership and bargaining committee, bargains with the employer on behalf of union members (rank and file members) and negotiates labour contracts (collective bargaining) with employers. The most common purpose of these associations or unions is "maintaining or improving the conditions of their employment". This may include the negotiation of wages, work rules, occupational health and safety standards, complaint procedures, rules governing status of employees including promotions, just cause conditions for termination, and employment benefits.

Unions may organize a particular section of skilled workers (craft unionism), a cross-section of workers from various trades (general unionism), or attempt to organize all workers within a particular industry (industrial unionism). The agreements negotiated by a union are binding on the rank and file members and the employer and in some cases on other non-member workers. Trade unions traditionally have a constitution which details the governance of their bargaining unit and also have governance at various levels of government depending on the industry that binds them legally to their negotiations and functioning.

Originating in Great Britain, trade unions became popular in many countries during the Industrial Revolution. Trade unions may be composed of individual workers, professionals, past workers, students, apprentices or the unemployed. Trade union density, or the percentage of workers belonging to a trade union, is highest in the Nordic countries.