A serve hit with minimal spin, relying primarily on speed. The ball travels in a straighter line, which means the margin for error is smaller but the pace is maximal. This is your big first serve. The serve you check the speed gun for. The tradeoff is that without spin to pull the ball down, you need height and angle working in your favor.
"Flat serve at 135 mph right up the T. The returner was guessing the other way. Ace."