A serve aimed at the center service line (the 'T' where the service line meets the center line). The shortest path from server to service box, which means maximum speed and minimum time for the returner. A flat serve down the T is the classic ace setup. The 'T' gets its name from the T-shape formed by the center service line and the service line.
"First serve right down the T at 130 mph. The returner barely flinches. Ace."