'Eldest' definitions:

Definition of 'eldest'

adjective

First in order of birth; "the firstborn child" [syn: firstborn, eldest]

noun

The offspring who came first in the order of birth [syn: firstborn, eldest]

From: GCIDE

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]