What is the greek word for well done?

There isn't one single Greek word that perfectly translates to "well done" in English, as the best translation depends on the context. The meaning could refer to:

* Excellent work: Words like άριστα (arista) or καλά (kala) (good/well) could be used, depending on the formality. Άριστα is more emphatic, suggesting "excellent" or "perfectly done."

* A job well completed: Ολοκληρώθηκε άριστα (oloklhróthike arista) – literally "it was completed excellently" – would be a more formal and complete phrase.

* Food cooked well: For cooked meat, ψημένο καλά (psimeno kala) – "well cooked" – would be appropriate.

So, there's no single perfect equivalent. You need to choose the phrase that best fits the specific situation.