adj. superlative form of old: most old.; greatest in age or seniority.
the GNU version of the Collaborative International Dictionary of English
adj. Oldest; longest in duration.
adj. Born or living first, or before the others, as a son, daughter, brother, etc.; first in origin. See Elder.
The Century Dictionary and Cyclopedia
Oldest; most advanced in age; that was born first: as, the eldest son or daughter.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. the offspring who came first in the order of birth
adj. first in order of birth
Word Usage
"Usually, the 2nd, 3rd and 5th sons are sprawled on our bed while watching cartoons on cable TV; the eldest is engrossed on highstreet 5 while the 4th son plays mafia wars on facebook."