What is the exact meaning of a "Nana" to you?

[C]

I have a friend around 45 that has a grand daughter. She posted a picture of her on Facebook. I told my friend; "Wow, she is so lucky to have a grandmother like you." She responded by saying; "Oh nooo I am her Nana." She wasn't upset .. but now I'm confused. I have always referred to my mothers mom as my grandmother .. never really used the world Nana but I have heard others use this title out of respect in their household. Just been wondering .. thanks for any input.

[JOЯDAN]

something you would call your grandmother.

[colossalwren256]

Well I call my grandmother nana so i think it means grandmother? at least to me it does.