[Sendric]

RotW is indeed newer, and it has a more detailed description.

Complete Warrior:

Lightblade, Elven: This rapierlike weapon is the size of a short sword, but weighs only as much as a dagger. Dexterous elf fighters and rogues favor it. Its thin, flexible blade slips easily into the seams of armor or between the ribs of a foe. Some elf nobles carry a lightblade - often decorated with intricate filigree and tiny gemstones - as a sign of their station, even if they aren't proficient in its use.

Races of the Wild:

Sword, Elven Lightblade: This rapierlike exotic weapon is the size of a short sword but weighs only as much as a dagger. Dexterous elf fighters and rogues favor it. Its thin, flexible blade slips easily into the seams of armor or between the ribs of a foe. Some elf nobles carry a lightblade - often decorated with intricate filigree and tiny gemstones - as a sign of their station, even if they aren’t proficient in its use.

Characters proficient with the elven lightblade may treat it as a rapier or a short sword for the purpose of any of the following feats: Greater Weapon Focus, Greater Weapon Specialization, Improved Critical, Weapon Focus, and Weapon Specialization.