The bat'leth, or "sword of honor", was a traditional Klingon blade weapon.
Resembling a crescent-shaped, two-ended scimitar, the bat'leth was wielded using three handholds along the outside edge of the blade. It was widely considered the most popular weapon among Klingon warriors.
According to Klingon mythology, the first bat'leth was forged by Kahless the Unforgettable in the 9th century. Kahless cut a lock of his hair and dropped it into the lava of the Kri'stak volcano, then plunged the burning lock into the Lake of Lusor and twisted it into a blade. After forging the weapon, he used it to fight the tyrant Molor, and then gave it its name. This story was not recorded in public texts, but was passed down verbally by the Klingon clerics as a test of Kahless' return. (TNG: "Rightful Heir")
The Sword of Kahless was preserved by the Klingons following Kahless' death, until it was stolen by the Hur'q when they plundered Qo'noS about 500 years after his death. (DS9: "The Sword of Kahless")
Worf owned a bat'leth that had been in his family for ten generations. After his mate K'Ehleyr was murdered by Duras, Worf used this weapon to kill Duras in honorable combat. (TNG: "Reunion")
During his time aboard the USS Enterprise-D, Worf trained both William T. Riker and Beverly Crusher in the use of the bat'leth. (TNG: "The Quality of Life", "The Pegasus")
A bat'leth was displayed among other trophies in the early-2280s mountain cabin of James T. Kirk, as re-created by the Nexus. (Star Trek Generations)
A traditional bat'leth was typically approximately 116 centimeters long, with blades of composite baakonite, weighs about 5.3 kilograms, and has an exterior hand grip diameter of 5 centimeters. (DS9: "Blood Oath")
The bat'leth was originally designed by martial arts expert Dan Curry, who also helped develop the intricate moves performed in its use. (Star Trek Encyclopedia)
The Klingon word for this weapon is betleH, etymologically derived from batlh 'etlh ("glory blade").
The Bat'leth appeared in the background in the Stargate SG-1, sixth season episode "The Other Guys". John Billingsley, who played Dr. Phlox on Enterprise, also appears in the episode, as a Trek-obsessed scientist.