A scab is a worker who crosses a picket line in violation of a planned work stoppage or strike. The term is used as a derogatory term for someone who leaves or declines to join a labor union, freeing them to work during a strike. Scabs are usually individuals who were not employed by the company before the trade union dispute but hired after or during the strike to keep the organization running. They may also refer to workers (union members or not) who cross picket lines to work. The use of strikebreakers is a worldwide phenomenon; many countries have passed laws outlawing their use to give more power to unionized workers.

The term "scab" has a long history and has been used as an insult to refer to workers who refused to join a union or acted in a way that would undermine their fellow workers. The term has a negative connotation and invokes a long, bloody history of violence against workers, in which private security forces, police, and even the National Guard have been deployed to quell labor unrest.

In summary, a scab is a worker who crosses a picket line during a planned work stoppage or strike, and the term is used as a derogatory term for someone who leaves or declines to join a labor union, freeing them to work during a strike.