Tannins.

Tea polyphenols, though popularly known as tannins, are not, as widely believed, the tannic acid used in leather preparation. In fact, they are not tannins at all. With some chemical and functional similarity, they became known by this name long before modern methods of chemical analysis made it possible to distinguish the two.

Here we have run into another of the name confusions that seem to haunt tea. There's more. Three-fourths of the tea polyphenols are catechins which are part of the chemical group flavanols (they do indeed have something to do with flavor, as distinct from either aroma or astringency). At least six catechins have been isolated.

We might make a summary outline like this:

Polyphenols (compounds with two or more phenolic hydroxyl groups)
Flavonols (one kind of polyphenol)
Catechins (the name for flavanols in tea)

Reports on the health benefits of these substances may use any of the three. Often no distinction is made between the terms 'catechins' and 'polyphenols,' and in many cases the two are used interchangeably.

(p. 88)

The polyphenols, about thirty altogether, account for nearly a third of the
soluble matter in the fresh tea leaf. During the fermentation process about a third (some say half) of the total amount is oxidized into more complicated oxidized products such as theaflavin. Therefore, after this process the tea contains two kinds of polyphenols, oxidized and unoxidized (natural polyphenols). The latter, released in the beverage, create the astringent, 'puckery' feeling in the mouth when you drink tea. This stimulates the salivary glands, which is why tea is a thirst quencher.

The unoxidized polyphenols provide the pungency, while the oxidized ones give the tea its color and flavor. The higher the degree of oxidation, the more color and less pungency a tea has. Green tea, which does not undergo oxidation, has more natural unoxidized polyphenols, and also more astringency. Black tea has more color but less astringency. Both oxidized and unoxidized polyphenols may be beneficial.

Now let's see what happens in the polyphenol oxidation process. At the tea factory, the freshly picked leaves sit until they have become soft and limp as a result of water evaporation. Then they are rolled to break down the membranes and bring the juices containing polyphenols in contact with the enzyme polyphenolase (polyphenol oxidase) which catalyzes the oxidation of the polyphenols by oxygen in the atmosphere. The product resulting from this process, together with other constituents, accounts for the unique flavor and rich, deeper color of black and oolong teas.

The action of the enzyme, and therefore the oxidation process, is eventually stopped through heating, but these compounds remain in the dried prepared leaves waiting for boiling water to dissolve them.