The traditional "sword of honor" preferred by Klingon warriors adept at the martial arts. About a meter long and shaped as a double-semicircle with four points (resembling a two-ended scimitar), the bat'leth is carried along the inside of the arm and wielded using two handholds on the outside edge of the weapon. A modern bat'leth is typically 116 centimeters long and weighs 5.3 kilograms, and is composed of baakonite metal.
Klingon oral history has it that the first bat'leth was forged around 625 A.D. (Terran equivalent) by Kahless the Unforgettable. Kahless dropped a lock of his hair into the lava from the Kri'stak Volcano, and then plunged the fiery lock into the lake of Lursor and twisted it to form a blade. After forging the weapon, he used it to defeat the tyrant Molor, and in doing so united the Klingon Homeworld. (Notation: This story was never recorded; rather, the clerics of Boreth passed the tale down among their sect through the generations to keep as a test of Kahless' prophesied return, as only Kahless would know this story outside of the clerics themselves.)
This first bat'leth was known as the Sword of Kahless, which was a sacred icon of the Klingon people until it was stolen about 1000 years ago by the invading Hur'q. In 2372, then-ambassador Kor set off with Worf and Jadzia Dax to the Gamma Quadrant to search for the legendary artifact, but came back empty-handed. (Notation: Officially, the Sword of Kahless remains lost, but rumors have it that Kor, Worf and Dax actually did find the ancient bat'leth, but opted to abandon it in free space, fearing that its return would only serve to divide the Klingon Empire.)
Worf, Son of Mogh, owns a bat'leth that has been in his family for 10 generations, which he used to kill Duras after the nemesis had murdered K'Ehleyr, the mother of Worf's son.
D'Ghor and Quark were both armed with the bat'leth when they faced off in the Great Hall before the High Council.
Koloth practiced using the ancient, crescent-shaped sword/shield in a holosuite program before the blood oath quest against the Albino Klingon -- and through an actual duel with Jadzia Dax, decided she can hold her own and come along.
The Jem'Hadar soldier Third Talak'talan knows all about Klingons and their sword of honor, and was disappointed not to meet one upon his first encounter with Alpha Quadrant natives.
References: Reunion, Rightful Heir, The Sword of Kahless,