The Collaborative International Dictionary of English v.0.48:

Rare \Rare\, a. [Compar. Rarer (r[^a]r"[~e]r); superl.
   Rarest.] [Cf. AS. hr[=e]r, or E. rare early. [root]18.]
   Nearly raw; partially cooked; not thoroughly cooked;
   underdone; as, rare beef or mutton.
   [1913 Webster]

         New-laid eggs, which Baucis' busy care
         Turned by a gentle fire, and roasted rare. --Dryden.
   [1913 Webster]

   Note: This word is in common use in the United States, but in
         England its synonym underdone is preferred.
         [1913 Webster]

WordNet (r) 3.0 (2006):

underdone
    adj 1: insufficiently cooked [syn: half-baked, underdone]