What is the greek word for granddaughter?

There isn't one single perfect equivalent in Greek for "granddaughter," as the word would depend on the gender of the granddaughter's parent. You'd need to use a phrase. For example:

* θύγατηρ τῆς θυγατρὸς (thygater tēs thygatrós) - daughter of the daughter. This is the most straightforward and commonly used way to say granddaughter.

* θύγατηρ τοῦ υἱοῦ (thygater tou huiou) - daughter of the son. This would be used if referring to a son's daughter.

So, there's no single word, but these phrases accurately convey the meaning.