What's a Yia Yia?

When Eleni tells her friend Luca about her Yia-Yia, he has no idea what she's talking about. That is until he realises he has one too - a grandmother Only his is called Nonna. In fact, all their friends have a grandmother - the only difference is the special name they have for her.

Grandchildren have different names for their grandmothers. What do you call your grandmother?