This is the 526th most frequent Greek word.
ελληνικής
"ελληνικής" means "of Greek" or "Greek" (possessive/descriptive form).
In this sentence, the word 'ελληνικής' is an adjective modifying 'γλώσσας', indicating the origin or identity of the language being referred to.
Η ιστορία της ελληνικής γλώσσας είναι μακραίωνη.
The history of the Greek language is centuries-old.
Here, 'ελληνικής' is an adjective that specifies the type of writing being discussed, attributing it to Greek origin or authorship.
Σπουδάζω λογοτεχνία ελληνικής συγγραφής.
I study literature by Greek authors.
In this context, 'ελληνικής' qualifies 'δημιουργίας' to describe that the music stems from Greek creativity or production.