n.1. a. An identical twin or unrelated person who has a similar appearance to a decent or moral person but who wreaks havoc or does evil.
b. An imaginary double humorously invoked to explain or excuse uncharacteristic or reprehensible behavior.
2. A negative or malign counterpart of something considered to be positive or good: "Not every drug's side effects are caused by a molecular evil twin" (Audrey Choi).
3. A fraudulent wireless internet access point used to commit identity theft by recording the data traffic of unwitting users.