The word scab gets tossed around loosely sometimes, but in industries which tend to use organized labor it means, specifically, an employee who contunues to report for work while a strike is officially in progress.

The scab could be a regular employee of the company, and a member of the union on strike, or he could be an outsider brought in by the company specifically to work during the strike.

Scabs are usually considered the lowest form of scum in unionized work forces and are subject to being fired after the strike, denied future employment, disrespect and humiliation at the hands of other union workers, and in the case of certain blue-collar trades, retaliatory violence.

In the airline industry there is a master scab list that goes back to the 1930's, and is widely circulated among airline pilots.