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]

Easton's 1897 Bible Dictionary:

First-born
   sons enjoyed certain special privileges (Deut. 21:17; Gen.
   25:23, 31, 34; 49:3; 1 Chr. 5:1; Heb. 12:16; Ps. 89:27). (See BIRTHRIGHT.)

     The "first-born of the poor" signifies the most miserable of
   the poor (Isa. 14:30). The "church of the first-born" signifies
   the church of the redeemed.

     The destruction of the first-born was the last of the ten
   plagues inflicted on the Egyptians (Ex. 11:1-8; 12:29, 30).

     Menephtah is probably the Pharaoh whose first-born was slain.
   His son did not succeed or survive his father, but died early.
   The son's tomb has been found at Thebes unfinished, showing it
   was needed earlier than was expected. Some of the records on the
   tomb are as follows: "The son whom Menephtah loves; who draws
   towards him his father's heart, the singer, the prince of
   archers, who governed Egypt on behalf of his father. Dead."