Mullerian mimicry is a biological phenomenon whereby two harmful species, which may not be closely related, come to mimic each other in their external appearance to scare away predators. These animals may have a common predator, and therefore experience mutual gain when their body patterns are associated with danger in the eyes of the predator, causing them to be passed by. The most commonly cited example of Mullerian mimicry is in butterflies, various lineages of which have similar colorful patterns on their wings to help scare away predators. The butterfly's actual repel mechanism is its foul taste.
Mullerian mimicry is one of many forms of mimicry employed by organisms to help them survive. The basis of many types of mimicry is aposematism -- the strategy whereby dangerous organisms (wasps, poison frogs, etc.) signal their defenses to predators through gaudy colors like bright yellow, orange, purple, or red. This is a strategy naturally opposed to crypsis, where the organism attempts to survive by attracting as little attention as possible, as in camouflage. Some organisms even employ both, trying to look inconspicuous until they are noticed, at which point they flash warning colors or symbols. This dual strategy is found among many snakes and amphibians.
The concept of Mullerian mimicry was first proposed in 1878 by Fritz Muller, a German naturalist and early proponent of Darwin's theory of evolution. In the immediate decades after the theory was published, naturalists spent a lot of time trying to explain certain seeming holes in the theory, reconciling it with field observations. A British naturalist, William Bates, studied Brazilian butterflies and came up with the concept of Batesian mimicry, whereby a harmless species mimics a harmful species, fooling predators into believing it is harmful. This showed how independently evolving lineages could come to resemble one another through natural selection. What was confusing were why harmful species came to resemble one another as well. This question was answered by Muller with his proposal of Mullerian mimicry.