Also known as Ve-Tsin, "accent" or MSG. Often added to food as a 'taste enhancer'.
The Chinese consider that there are five fundamental tastes: salt, sweet, sour, bitter and 'savory'. The Japanese call this taste umami (OO-mommy). MSG would be the defining example of savory, in the same way that sugar is the defining example of sweet.
Apparently, a Japanese scientist demonstrated that we have taste buds for glutamate. However, the existence of umami is by no means universally accepted.
Apparently some people can be allergic or sensitive to MSG, causing migraines and other discomfort. This has led to wide-spread rumours (compare: NutraSweet, DihydrogenMonoxide) about MSG causing something called the 'China Restaurant Syndrome'. We have sodium ions and glutamate ions in us at all times, so it is not known whether MSG really is dangerous, and if so, why.
And now for something CompletelyDifferent:
The council of a small town called upon the services of a noted herpetologist to study the decline in numbers of the amphibian population at the local pond. The herpetologist arrived during breeding season, and immediately noticed that the males were slipping off the females whenever they attempted to mount. He whipped up a concoction to add to the waters of the pond that contained two parts of this and three parts of that, but most critically, exaclty one part of sodium. The chairman of the council groaned at the inevitable punchline that was about to be delivered: "Oh no! Surely you don't mean...?" "That's correct," replied the herpetologist. "They need a monosodium glue to mate."
See also: WorstPunEver