Astringency is a sensation you feel within your mouth when you drink some teas. This sensation is caused mainly by polyphenols, which are anti-oxdants. These include both the catechins of green teas and tannins that give black tea its color. It is the fuzzy and dry feeling you get on your tongue which is caused by the chemicals binding to the proteins in saliva causing the tissues of the mouth to constrict. Preference for astringency is a matter of personal taste. Next time you have a cup of green or black tea, note whether you like this sensation or not.