What is the greek word for godmother?

There isn't a single, universally accepted Greek word for "godmother." The concept is expressed using descriptive phrases. You would likely use something like νονά (noná), which is more commonly used to mean "godmother" in modern Greek, even though it's literally a diminutive of "mother" (μητέρα, mitéra) and implies a close, maternal relationship rather than strictly a religious one.