The fifth taste, the other four being salty, sweet, sour and bitter. "Discovered" and named by Kikunae Ikeda around the beginning of the 20th century. Nirupa Chaudari of the Miami School of Medicine published in February, 2000 in "Nature Neuroscience" a study that proves its existence. Some tastebuds have a molecule which acts as a receptor for L-glutamate, which is a marker for high-protein foods, and thus sought out by many animals including humans. MonoSodium Glutamate (msg) is of course a tasty source of umami but so is parmesan cheese.

The reason why this word is not a part of the English language is that there is no word to describe such a taste in the English language. The closest to umami that I can think of to describe in English is savory, yummy, tasty. But none of these are quite it.

It remains to be seen whether scientists will ever agree on the existence of other taste elements besides these five; astringency and especially fat are considered strong contenders. The idea that there are only four held on for an amazingly long time, when you consider how flavourful mushrooms can be, for instance, without being notably sweet, sour, salty or bitter, and without having such a strong smell that you can plausibly explain their taste that way.

Meat meals often taste pretty umami because the presence of lots of protein is strongly correlated with glutamate, which is behind the umami taste. For vegetarian meals, it's often a good idea to include a natural source of monosodium glutamate to fill out the flavour. For example, mushrooms, celery, soy sauce and vegetable stock are all rich sources of MSG.