Bon après recherche ça se discute quand même:

-à l'origine le nom veut dire "descendants de Shem" donc bon décrit a priori une ethnie
-en pratique:

Modern science, in contrast, identifies a population's common physical descent through genetic research, and analysis of the Semitic-speaking peoples suggests that they have some common ancestry. Though no significant common mitochondrial results have been yielded, Y-chromosomal links between Semitic-speaking Near-Eastern peoples like Arabs, Assyrians and Jews have proved fruitful, despite differences contributed from other groups (see Y-chromosomal Aaron). Although population genetics is still a young science, it seems to indicate that a significant proportion of these peoples' ancestry comes from a common Near Eastern population to which (despite the differences with the Biblical genealogy) the term "Semitic" has been applied.

Évidement c'est débile de considérer les juifs 'exotiques' comme sémites de ce point de vue (ils ont 1/2^20 de sang originaire du coin ..), ce qui retire déjà bcp de problèmes d'homogénéité. Et en gros ça dépeint une famille de peuples ( d'ethnies ) au langage et à la génétique assez proche.