A tannin (also known as vegetable tannin, natural organic tannins, or sometimes tannoid, i.e. a type of biomolecule, as opposed to modern synthetic tannin) is an astringent, bitter plant polyphenolic compound that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids.

The term tannin (from tanna, an Old High German word for oak or fir tree, as in Tannenbaum) refers to the use of wood tannins from oak in tanning animal hides into leather; hence the words "tan" and "tanning" for the treatment of leather. However, the term "tannin" by extension is widely applied to any large polyphenolic compound containing sufficient hydroxyls and other suitable groups (such as carboxyls) to form strong complexes with various macromolecules.

The tannin compounds are widely distributed in many species of plants, where they play a role in protection from predation, and perhaps also as pesticides, and in plant growth regulation.
Source: https://en.wikipedia.org/wiki/Tannin (derived 1-4-2016)

We have an ancient relationship with tannin, as we know it from our food. Think about a bite of an unripe apple or a sip of sturdy red wine. It is the tannin that gives us that rough feeling in our mouth. It astringes the proteins from which our mucosa consists. In large this is the same thing that happens to the skin proteins when we tan them into leather.