Mirin is a sweet golden-yellow wine used in Japanese cooking as a sweetener. It's more refined and mellower than table sugar and has a distinctive fragrance. When used in a basting sauce, mirin gives meats and fish an appealing gloss. In a marinade, mirin tenderizes, in addition to providing flavor.
Mirin was once drunk as an apéritif, just as the Spanish sip sherry. This was back when mirin was made artisanally with authentic ingredients, including glutinous rice. Today, most mirin is made from other starches and is mass-produced, making it unsuitable for drinking straight.
A substitute for mirin is 1 Tbs. sake or white wine plus 2 tsp. sugar.
The best mirin may be labeled hon-mirin: honjozo (which means "true mirin: naturally brewed"). Even though this isn't "real" mirin in the artisanal sense, it does retain some of the traditional processing steps. Lower quality mirin is sold under the name mirin-fu chomiryo ("a kind of mirin") and aji-mirin ("mirin taste"). Mirin is usually available in the Asian foods section of well-stocked supermarkets, but you'll find the best selection at a specialty market.
Refrigerated, mirin will retain its flavor for two months.