Usually some type of machine consturcted (by evil) for the purpose of:

  1. Destroying The World
  2. Ruling The World
  3. Killing people
    or;
  4. Enslaving (ruling) people

As a general rule, all inventions of evil villains must have the suffix "-tron" or "-tor", and/or be a ray of some sorts. For example: The orgasmotron, megatron doomsday machine, the annihilator, the bikini ray, pimp ray, etc.