someone that goes to work at a company with workers on strike and takes a position that is represented by the striking union.
the bottle washers union is on strike at the acme bottle company. the bottle washers union only represents the bottle washers. there are other unions on the property like the bottle cappers union, but they're not on strike.
bob and larry get hired by acme bottle company. bob goes to work as a bottle washer and larry takes a job as a bottle capper. bob is a scab, larry is not.