This is the 346th most frequent Greek word.
παναθηναϊκός
The Greek word "παναθηναϊκός" refers to something "relating to all Athenians" or the Panathenaic festival; it can also denote the Panathinaikos sports club.
In this sentence, "Παναθηναϊκός" is used as a proper noun referring to a Greek sports club.
Ο Παναθηναϊκός παίζει σήμερα έναν κρίσιμο αγώνα.
Panathinaikos is playing a crucial match today.
Here, "παναθηναϊκούς" is used as an adjective describing the fans related to Panathinaikos.
Το στάδιο είναι γεμάτο από παναθηναϊκούς οπαδούς.
The stadium is full of Panathinaikos fans.
In this context, "Παναθηναϊκού" is in the possessive form, indicating belonging to Panathinaikos.