Where does “Πανέλληνες” come from?

Πανέλληνες (Ancient Greek) comes from Ancient Greek Ἕλλην, from Ancient Greek Ἑλλοί, from Ancient Greek ἕλλα, from Proto-Hellenic *héllā.

Πανέλληνες (Ancient Greek): all Greeks, the Greek World

Definitions

Step	Language	Word	Meaning
1	Ancient Greek	Ἕλλην	Greek, one who is from Greece or speaks Greek;...
2	Ancient Greek	Ἑλλοί	The Greeks in Dodona
3	Ancient Greek	ἕλλα	seat
4	Proto-Hellenic	*héllā	—