Astringency is one of the things our mouths detect which falls outside of the five main flavours recognised by science, but which undoubtedly makes a big difference to the way things taste. Indeed, Ayurveda counts it as one of the six tastes alongside sweet, salt, sour, bitter and pungent (the first four of these correspond to the four tastes long recognised as science as categories of gustation, to which a fifth, the savoury flavour known as umami, was added relatively recently). Astringency is the property of things like tea (especially black tea) and wine (especially red wine) which causes your mouth to pucker, often accompanied by a dry, rough feeling. Fruit and vegetables are also commonly astringent - think of cranberries. If something is too astringent, it becomes impossible to eat or drink - but just a little astringency is pleasing to the soft palate and gives a drink body; it also helps to clear the palate after eating foods high in oil or protein.

Astringency in food and drink is thought to be caused by astringent chemicals binding with proteins in saliva. Like bitterness, it is often associated with chemicals which would be poisonous in large amounts, but are very good for your health in the quantities found in fruit, vegetables and tea - chiefly antioxidants such as flavonoids, notably the tannins. Unfortunately much of the effort put into breeding food plants in the twentieth century went into removing these chemicals from vegetables, in order to make them as appealing as possible to the mass market - so today's vegetables tend to have much diminished antioxidant power compared to their less intensively cultivated ancestors. To make bitter and/or astringent vegetables more palatable, it often helps to serve them with oil and a little salt - a method which is ubiquitous in Mediterranean cooking.

Adding milk to black tea is one way to take the edge off its astringency; combining rice tea with green tea as in Genmai-cha has a similar effect. Lemon juice also makes black tea less astringent, by breaking down the tannin, but adds some astringency and plenty of sourness of its own. I've found that drinking straight black tea or lemon tea in the morning after heavy drinking makes me throw up. I suspect this is due to its astringency, but I can't back that up with any hard science. Various sources suggest that the astringency of assorted herbs makes them useful for the treatment of a range of complaints from diarrhoea to skin inflammation - I'm not sure how much that is supported by science, either.

Fascinating coverage of taste in general, with special reference to the contribution of trigeminal sensations to the overall effect:
Astringency in wine and grapes:
More on the chemistry of wines:
And more (abstract only):
Bitterness, astringency and health: (based on December 2000 issue of American Journal of Clinical Nutrition)
More on phytochemicals:
With thanks to BlueDragon for advice and useful links.