Wikipedia
P.O.K. (from the initial letters of Panathinaikos, Olympiacos and Konstantinoupolis-AEK-) was a coalition, a kind of an athletic trust, of the three main football teams of the Greater Athens area (the "big three") which lasted until the mid 60's. The term dates to in 1927, when Olympiacos F.C., Panathinaikos F.C. and AEK F.C. withdrew from the Greek Championship after disagreements with the Hellenic Football Federation (HFF), mainly over the championship's financial status. The HFF determined that league's revenues would be equally divided between all teams that participated. Olympiacos, Panathinaikos and AEK did not agree with this, and formed a group called P.O.K. During that season, they played friendly matches with each other.
The name P.O.K. is currently used to allude to the continuing dominance of the three clubs in the National A Division. Rarely does a team outside these three finish in the top three spots of the Championship; doing so is called "breaking the P.O.K.". Since 1928, only three other teams - Aris F.C. (Thessaloniki), AE Larissa and PAOK F.C. - have won the Greek Championship. The last-named is considered the main team that has "broken the P.O.K.", most recently having finished second in the 2008/09 season, leaving Panathinaikos and AEK in third and fourth places.