Aposematism (from Greek ἀπό apo away, σ̑ημα sema sign) was a new term coined by Edward Bagnall Poulton for Alfred Russel Wallace's concept of warning coloration. It describes a family of antipredator adaptations in which a warning signal is associated with the unprofitability of a prey item to potential predators. Aposematism always involves an advertising signal. The warning signal may take the form of conspicuous animal coloration, sounds, odours or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, since both avoid potential harm.
Aposematism is exploited in Müllerian mimicry, where species with strong defences evolve to resemble one another. By mimicking similarly coloured species, the warning signal to predators is shared, causing them to learn more quickly at less of a cost to each of the species.
Warning signals do not necessarily require that a species actually possesses chemical or physical defences to deter predators. Mimics such as the nonvenomous California mountain kingsnake (Lampropeltis zonata), which has yellow, red, and black bands similar to those of highly venomous coral snake species, have essentially piggybacked on the successful aposematism of the model. The evolution of a warning signal by a mimicking species that resembles a species that possesses strong defences is known as Batesian mimicry.
Some wonderful terms in there like aposematism, Müllerian mimicry, Batesian mimicry. I recently wrote about the value of using rhetoric to encode one's ideas. German is gloriously efficient in its compression, with single words like schadenfreude for phenomenon that require many more words in English, but fields like science, philosophy, psychology, and sociology offer their own concise gems of language.
The rest of the entry is a good read on why aposematism might have come to be despite seeming to be an evolutionary paradox, and just what Müllerian and Batesian mimicry refer to.
What intrigues me about aposematism is how it functions as a really overt form of signaling. I find myself reaching for the framework of signaling theory often these days, perhaps because once you've lived a certain number of years, you've had a chance to see the relative effectiveness or futility of past signals, but you're not yet old enough to be at the point where your remaining days are so few or your status in the world so static that signals no longer matter.