Tannin describes astringent, mouth-drying compounds — extracted from apple skins and seeds during cidermaking, or from oak during barrel aging in any beverage — that contribute real structure and complexity rather than simply reading as a flaw the way excess astringency in beer usually does.
Why cider treats tannin as a feature, and beer treats it as a mistake
This is a genuine point of contrast between beer and cider brewing practice on this site: astringency from over-hot sparge water or crushed grain husks is a defect in beer, something every mashing and sparging page here warns against. In cider, tannin from bittersharp and bittersweet cider-specific apple varieties is a deliberately sought structural element — the reason those traditional cidermaking apple varieties, distinct from sweet culinary or dessert apples, matter so much to a traditional cider's finished character.
Where else tannin shows up in homebrewing
Oak aging, whether from barrels or oak cubes/spirals added to a fermenter, is the other common source of deliberate tannin extraction — most relevant to long-aged, strong styles on this site like Barleywine or Belgian Dark Strong Ale, where a brewer might use limited oak contact specifically to add structural complexity in the same spirit cider apples do naturally.