Rice vinegar
A mild, slightly sweet vinegar made from fermented rice.
Rice vinegar is noticeably milder and less sharp than wine or distilled vinegar. It is sold in Asian grocers and in the international aisle of larger supermarkets; seasoned rice vinegar already contains sugar and salt, so look for the unseasoned kind.
Substitute:
White wine vinegar or cider vinegar, diluted with a little water and softened with a pinch of sugar, comes close; the result is a touch sharper and fruitier.