The Collaborative International Dictionary of English v.0.48:

Explorer \Ex*plor"er\, n.
   One who explores; also, an apparatus with which one explores,
   as a diving bell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

explorer
    n 1: someone who travels into little known regions (especially
         for some scientific purpose) [syn: explorer,
         adventurer]
    2: a commercial browser [syn: Internet Explorer, Explorer,
       IE]