The Collaborative International Dictionary of English v.0.48:

Rat \Rat\ (r[a^]t), n. [AS. r[ae]t; akin to D. rat, OHG. rato,
   ratta, G. ratte, ratze, OLG. ratta, LG. & Dan. rotte, Sw.
   r[*a]tta, F. rat, Ir. & Gael radan, Armor. raz, of unknown
   origin. Cf. Raccoon.]
   1. (Zool.) One of several species of small rodents of the
      genus Rattus (formerly included in Mus) and allied
      genera, of the family Muridae, distinguished from mice
      primarily by being larger. They infest houses, stores, and
      ships, especially the Norway rat, also called brown rat,
      (Rattus norvegicus formerly Mus decumanus), the black
      rat (Rattus rattus formerly Mus rattus), and the roof
      rat (formerly Mus Alexandrinus, now included in Rattus
      rattus). These were introduced into America from the Old
      World. The white rat used most commonly in laboratories is
      primarily a strain derived from Rattus rattus.
      [1913 Webster +PJC]

   2. A round and tapering mass of hair, or similar material,
      used by women to support the puffs and rolls of their
      natural hair. [Local, U.S.]
      [1913 Webster]

   3. One who deserts his party or associates; hence, in the
      trades, one who works for lower wages than those
      prescribed by a trades union. [Cant]
      [1913 Webster]

   Note: "It so chanced that, not long after the accession of
         the house of Hanover, some of the brown, that is the
         German or Norway, rats, were first brought over to this
         country (in some timber as is said); and being much
         stronger than the black, or, till then, the common,
         rats, they in many places quite extirpated the latter.
         The word (both the noun and the verb to rat) was first,
         as we have seen, leveled at the converts to the
         government of George the First, but has by degrees
         obtained a wider meaning, and come to be applied to any
         sudden and mercenary change in politics." --Lord Mahon.
         [1913 Webster]

   Bamboo rat (Zool.), any Indian rodent of the genus
      Rhizomys.

   Beaver rat, Coast rat. (Zool.) See under Beaver and
      Coast.

   Blind rat (Zool.), the mole rat.

   Cotton rat (Zool.), a long-haired rat (Sigmodon
      hispidus), native of the Southern United States and
      Mexico. It makes its nest of cotton and is often injurious
      to the crop.

   Ground rat. See Ground Pig, under Ground.

   Hedgehog rat. See under Hedgehog.

   Kangaroo rat (Zool.), the potoroo.

   Norway rat (Zool.), the common brown rat. See Rat.

   Pouched rat. (Zool.)
      (a) See Pocket Gopher, under Pocket.
      (b) Any African rodent of the genus Cricetomys.

   Rat Indians (Ethnol.), a tribe of Indians dwelling near
      Fort Ukon, Alaska. They belong to the Athabascan stock.

   Rat mole. (Zool.) See Mole rat, under Mole.

   Rat pit, an inclosed space into which rats are put to be
      killed by a dog for sport.

   Rat snake (Zool.), a large colubrine snake (Ptyas
      mucosus) very common in India and Ceylon. It enters
      dwellings, and destroys rats, chickens, etc.

   Spiny rat (Zool.), any South American rodent of the genus
      Echinomys.

   To smell a rat. See under Smell.

   Wood rat (Zool.), any American rat of the genus Neotoma,
      especially Neotoma Floridana, common in the Southern
      United States. Its feet and belly are white.
      [1913 Webster]

WordNet (r) 3.0 (2006):

brown rat
    n 1: common domestic rat; serious pest worldwide [syn: brown
         rat, Norway rat, Rattus norvegicus]