A Universal Sword is a sword that has no restrictions based on race, class, alignment or size. A universal sword also requires no weapon proficiencies.
The prime example is the Silver Sword of Gith, used by the protagonist in the Neverwinter Nights 2 Official campaign and perfected in the expansion, Mask of the Betrayer.
Because it is a universal weapon, a character may not take Weapon Focus or Weapon Specialization in universal sword. However, Weapon Focus (Longsword) and Weapon Specialization (Longsword) feats will apply their effects and longsword Weapon Masters may use their special abilities with a universal sword.