Acts 2:5 - 2:6: What is an ἔθνος?

  In Acts 2:5 we read this:

ἄνδρες εὐλαβεῖς ἀπὸ παντὸς ἔθνους 

τῶν ὑπὸ τὸν οὐρανόν 

devout men from every nation under the heaven

Note that, (1) this form of ἔθνος, τό is neuter singular genitive, although the ending looks like it could be masculine plural accusative,

(2) this word is, indeed, related to the English 'ethnic',

(3) almost every English translation that I have seen translates 'ἔθνους' as 'nation' in this verse, but

(4) another common translation is 'Gentiles', i.e., people who are not Jewish. 

Indeed, 'nation' might be misleading in the New Testament, as the modern conception of this word is something like 'the entity called France, or India, or Canada', and so on.  'Nations' have borders, their own governments, possibly their own language/s and monetary system, and so on. 

The usage in the New Testament does not seem to be 'nation' as in 'France', but 'nation' as in 'an established group of people, bound by ties of language and/or tradition, sometimes living together'.  The term often suggests a contrast between Jewish and non-Jewish groups.

There is a trace of this meaning in the term that was often (although not always) used for the head of state when Greek was a monarchy: the 'King of the Hellenes' - i.e., of a group of people - as opposed to the 'King of Greece' - i.e., of a nation.

'Τὸ ἔθνος' is known from Homer down, and it did not necessarily even refer to human beings:

τῶν δ᾽ ὥς τ᾽ ὀρνίθων πετεηνῶν ἔθνεα πολλὰ

χηνῶν ἢ γεράνων ἢ κύκνων δουλιχοδείρων

as many flocks of birds in flight,

cormorants or geese or swans, with necks outstretched . . .   (Iliad, 2: 459)