A scapegoat was someone who took the blame for a mistake or tragedy.
In 2151, Jonathan Archer accused the Vulcan High Command of wanting T'Pol to be their scapegoat for the P'Jem incident, since they couldn't blame him. (ENT: "Shadows of P'Jem")
In 2373, when trying to convince Winn Adami to continue negotiations without Bareil Antos's help, Julian Bashir accused her of cowardice, wanting Bareil to be her scapegoat should the talks fail. (DS9: "Life Support")