Tuna
A large sea fish with firm, deep red to pink flesh, eaten raw, briefly seared or from the tin.
Tinned tuna and frozen steaks are available almost everywhere. Fresh tuna of a quality fit to be eaten raw is another matter: it is found only at good fish counters, fishmongers, well-stocked Asian shops or by mail order, and you should ask for it explicitly.
Substitute:
For raw dishes, salmon or yellowtail of raw-eating quality can stand in, both richer and milder; for cooked dishes swordfish works, though it turns drier.