The Collaborative International Dictionary of English v.0.48:

loupe \loupe\ (l[=oo]p), n.
   Any of a variety of small magnifying glasses, having a
   magnifying power from 2X to 20X, commonly of 10X, which may
   be held in the hand, or in some cases held within the
   eyesocket without the use of a hand. It is commonly used by
   jewelers and watchmakers.
   [PJC]

WordNet (r) 3.0 (2006):

loupe
    n 1: small magnifying glass (usually set in an eyepiece) used by
         jewelers and horologists [syn: loupe, jeweler's loupe]