Japanese rice vinegar. The mild and slightly sweet su may be used variously, such as a dressing for sunomono (vinegared food) and other foods, for seasoning, in acidulated water to retain color in vegetables and in sushi meshi, the rice used for sushi dishes. It's available in Asian markets and most larger supermakets.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.