The Collaborative International Dictionary of English v.0.48:

firstborn \firstborn\ n.
   The first child born to a parent; as, his firstborn was
   showered with gifts from relatives.

   Syn: eldest.
        [PJC]

The Collaborative International Dictionary of English v.0.48:

Firstborn \First"born`\, a.
   First brought forth; first in the order of birth; eldest;
   hence, most excellent; most distinguished or exalted; as, the
   firstborn child.

   Syn: eldest.
        [1913 Webster]

WordNet (r) 3.0 (2006):

firstborn
    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]