What is the greek word for telegraph?

There isn't a single, universally accepted ancient Greek word for "telegraph" because the technology didn't exist then. You would need to use a descriptive phrase, perhaps something like ηλεκτρικός τηλέγραφος (ēlekrikós tēlḗgraphos), which literally translates to "electric telegraph." This uses modern Greek words, but it would be understandable based on the components.