I believe it is a prefix meaning Θωρηκτό or Battleship.

There might have been differences in the meaning of battleship between Greece and other countries.

Further searching, looks like it's a prefix for Θωρακισμένο Καταδρομικό(Armored Cruiser) which were/are? popularly called Θωρηκτό(Battleship). From Greek cruiser Georgios Averof:

Georgios Averof (Greek: Θ/Κ Γεώργιος Αβέρωφ) is a Greek warship which served as the flagship of the Royal Hellenic Navy during most of the first half of the 20th Century. Although popularly known as a battleship (θωρηκτό), it is in fact an armored cruiser,[1] (θωρακισμένο καταδρομικό), the only ship of this type still in existence.