n :  any of the numerous rodents (family Muridae) of Rattus and related genera that differ from the murid mice by their usu. considerably larger size and by features of the teeth and other structures and that include forms (as the brown rat, the black rat, and the roof rat) which live in and about human habitations and in ships, have become naturalized by commerce in most parts of the world, and are destructive pests consuming or destroying vast quantities of food and other goods and acting as vectors of various diseases (as bubonic plague) .
 
 
 
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