Merlan is the traditional name in France. Elsewhere, butchers sell the same piece — or a close cousin — under a different name.
Third member of the *morceaux du boucher* trio — all three (poire / merlan / araignée) are French-butchery boucher's-secret cuts traditionally kept by the butcher.
Companion *morceau du boucher* — both small premium muscles on the round, both historically reserved by French butchers, both quick-cook tender cuts.