* Συνταξιούχος (syntaxiouhos): This literally means "pensioner" and is the most common and readily understood way to refer to someone who is retired. It emphasizes the receipt of a pension.
* Απόμαχος (apómachos): This means "discharged from service" or "retired from military service". While not strictly applicable to all retired individuals, it can be used figuratively.
* Σε ηλικία συνταξιοδότησης (se ilikiá syntaxiódótisis): This phrase means "at retirement age," describing someone's status rather than a state of being.
* You could also use descriptive phrases like έχει αποσυρθεί από την εργασία του (éχει aposyrthéi apó tin ergasía tou) which means "he/she has withdrawn from his/her work".
The best choice will depend on the context. For most situations, συνταξιούχος (syntaxiouhos) is the most appropriate and easily understood word.