An off-hand weapon is light enough that the attacker can hold it and attack effectively with it while also holding a weapon in his or her main hand. An adventurer can’t attack with both weapons in the same turn, unless he or she has a power that allows such an attack, but he or she can attack with either weapon.

Published in Player's Handbook, page(s) 217, Mordenkainen's Magnificent Emporium, page(s) 20, Rules Compendium, page(s) 270.