pickled ginger
Wafer-thin young ginger pickled in sweet rice vinegar, mildly hot and crisp.
Many larger supermarkets stock it on the Asian shelf, and Asian grocers always carry it in jars or pouches. Usually this means the pale pink sushi ginger (gari); the bright red, salty beni shoga cut in fine strips is a different pickle and not freely interchangeable.
Substitute:
It is easy to make at home: shave young ginger very thinly, blanch briefly and steep for a few hours in rice vinegar with sugar and salt. Raw fresh ginger is no substitute, as it is far hotter and lacks the sweet-sour note.