well, I think Alex has the close of it, but it does get used in a couple of different ways. there are also a couple of different music groups and djs and blogs, etc., etc.

the most interesting use I found was here: the lesson of butterflies

Quote:

Viceroy butterflies have long been thought to be the leading example of an evolutionary play known as Batesian mimicry. The Viceroy was thought to be a nontoxic copy of the Monarch. The coloration similarity increased the odds against being eaten. Thus members of this species take advantages of lessons predators have already learned about avoiding their toxic twin.

The case of the Viceroy and Monarch appears to be more complex. The red-winged blackbird is a natural butterfly predator. It was used in an experiment as a taste tester. The birds were fed the bodies of Viceroys, Monarchs and other butterflies known to be the blackbird's favorites. The birds eagerly ate 98 percent of the other butterflies but only 41 percent of the Viceroys and 46 percent of the Monarchs. This suggests that the Viceroys taste just as bad as the Monarchs. Since the wings had been removed, the similar color or Batesian mimicry is not all that has evolved. This experiment suggests that the two foul-tasting butterflies may have used another ploy to evolve into look-alikes. By doubling their numbers, these similar butterflies more quickly taught predators avoidance. The Monarch and Vicerory doubled their protection.



the first paragraph seems to imply that a "toxic twin" is one that carries some trait or behavior that affects others who are similar, though don't necessarily carry that same trait or exhibit the same behavior. the second paragraph however, muddies that idea for me, a bit.

so, I don't know that we're any closer to a "definitive" definition.

formerly known as etaoin...