Sweet, Sour, Salty, Bitter. Since Democritus and Aristotle, these have been known as the four tastes a person is capable of experiencing. But a century ago in Japan, a theory emerged about a fifth taste, which has been either unknown or actively denied in the West. It's called umami, and it translates as "savoriness," or "meatiness" or sometimes just "deliciousness." It is written thus: The Chinese know it as xiānwèi. The glutamates prevalent in protein-rich foods are thought to be the source of umami, which is why the addition of monosodium gluatamate, or ajinomoto, makes things taste more rounded and deeply savory.
In this illustration of the tongue, orange represents the place that senses bitterness, green sourness, blue saltiness and purple sweetness. At the center of it all is the yellow -- that's right, umami, that elusive, hard-to-define fifth taste. For more about the history of the discovery of umami, check out this recent Food program on NPR.
By the way, this raises all sorts of confusing questions about the art of French kissing. Which part of the tongue is capable of tasting another tongue? Perhaps there's a special French word for that.