It is called a service ace!
There ISN'T anything called a "backhand serve" in tennis.
If the player is unable to hit the serve then it is an ace. But if the player hits the ball but does not return it, then it is a forced error.
Because you hit the ball with some kind of racket and that is why you call it a serve.
The legal serve where the opponent fails to touch the tennis ball is called 'ACE'.
A missed serve is called a fault. When she/ he serve is a fault if the server swings and misses the ball