Tannins are a class of compounds in tea, especially black tea, which tend to have a bitter flavor and astringent properties. Teas high in tannins can be described as tannic.
Tannins are naturally occurring and common, and are an important component of red wine. They are also responsible for the dark color in some streams, as they are found in leaves and wood, and are released as organic matter breaks down.
The name "tannin" originated in the historical use of these compounds in tanning animal hides to make leather.