The Collaborative International Dictionary of English v.0.48:

Eldest \Eld"est\, a. [AS. yldest, superl. of eald old. See
   Elder, a.]
   1. Oldest; longest in duration. --Shak.
      [1913 Webster]

   2. Born or living first, or before the others, as a son,
      daughter, brother, etc.; first in origin. See Elder. "My
      lady's eldest son." --Shak.
      [1913 Webster]

            Their eldest historians are of suspected credit.
                                                  --Bp.
                                                  Stillingfleet.
      [1913 Webster]

   Eldest hand (Card Playing), the player on the dealer's left
      hand. --R. A. Proctor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eldest
    adj 1: first in order of birth; "the firstborn child" [syn:
           firstborn, eldest]
    n 1: the offspring who came first in the order of birth [syn:
         firstborn, eldest]