Ground chicken
Minced chicken meat, in Japanese cooking usually from the juicier thigh.
Ground chicken is common in North American supermarkets but less reliably stocked elsewhere; a butcher can mince it to order. The simplest route is to make it yourself by finely chopping skinless thigh meat with a knife or pulsing it briefly in a food processor.
Substitute:
Ground turkey is the closest substitute but turns out drier and slightly firmer; home-chopped chicken thigh matches the original best, while breast meat alone tends to be dry.