A trade union (British English) or labor union (American English), often simply referred to as a union, is an organisation of workers intent on "maintaining or improving the conditions of their employment", such as attaining better wages and benefits, improving working conditions, improving safety standards, establishing complaint procedures, developing rules governing status of employees (rules governing promotions, just-cause conditions for termination) and protecting and increasing the bargaining power of workers.

Trade unions typically fund their head office and legal team functions through regularly imposed fees called union dues. The union representatives in the workforce are usually made up of workplace volunteers who are often appointed by members through internal democratic elections. The trade union, through an elected leadership and bargaining committee, bargains with the employer on behalf of its members, known as the rank-and-file, and negotiates labour contracts (collective bargaining agreements) with employers.

Unions may organize a particular section of skilled or unskilled workers (craft unionism), a cross-section of workers from various trades (general unionism), or an 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.