An accessory was someone who helped a criminal commit a crime.
In 2152, Jonathan Archer suggested to Firek Plinn that, among other things, he could be charged with accessory to kidnapping, since his brother had kidnapped Kaitaama and later Charles Tucker. (ENT: "Precious Cargo")
In 2374, Benjamin Sisko described himself as accessory to murder in a (later deleted) personal log, as his operation with Garak regarding the Romulans led to Garak killing Grathon Tolar and Vreenak. (DS9: "In the Pale Moonlight")
In 2377, Kathryn Janeway refused to accept a polaron modulator from Bosaal, who had killed the crew of a Kinjal frigate for it, since using it would make the USS Voyager crew accessories to murder. (VOY: "The Void")