Smoked salmon is a blanket term for any salmon: wild, farmed, fillet, steak, cured with hot or cold smoke.
Lox refers to salmon cured in a salt-sugar rub or brine (like gravlax). Nova is cured and then cold-smoked (unlike lox or gravlax). There's also hot-smoked salmon, which is cured, then fully cooked with heated wood smoke.
Now, to acknowledge the purists. Real, authentic lox is made from only the belly portion of the salmon. Yup, like pork, the belly of the fish is typically the richest, fattiest and most succulent portion. Cured and smoked, it's saltier and more…uh…"aromatic" than its milder non-belly counterpart, and if you're lucky enough to try it on a bagel with cream cheese, it's hard to go back. When you buy lox anywhere other than an old-school appetizing counter, even if it's clearly labeled "lox," what you're almost certainly getting is simply smoked salmon. And frankly, that's fine by us.
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