Statement of authenticity
A statement of authenticity or letter of authenticity was a document that assured that an object was the genuine article it claimed to be, not a forgery or counterfeit.
In 2369, Quark promised Kolos that each Gamma Quadrant artifact he was selling came with a statement of authenticity from Vash, who the Ferengi described as "the Federation's foremost expert on the Gamma Quadrant" (DS9: "Q-Less")
In 2370, Odo questioned whether there was supposed to be a letter of authenticity on Plegg's vacuum-desiccated disks. Quark pointed him to the Ferengi Seal of Dismemberment. (DS9: "The Alternate")