beef liver
Liver from beef cattle, stronger and slightly more bitter in flavour than calf's or chicken liver.
Beef liver is usually found at a butcher's or a meat counter, often to order, and frozen in Turkish or Eastern European shops. It turns tough and grainy if cooked too long, and soaking it in milk for a while beforehand softens its flavour.
Substitute:
Calf's liver is more tender and milder and makes the best substitute; lamb's or pig's liver come closer to the strong flavour but differ somewhat in texture.