A squadron leader, or squad leader for short, was the lead member of a flight squadron.
In 2368, Nicholas Locarno led the elite Starfleet Academy flight squadron known as Nova Squadron, prior to the accident that resulted in the death of Joshua Albert. (TNG: "The First Duty")
In 2370, Santos was the squad leader of a Maquis squadron in an attack on a Yridian convoy. (TNG: "Preemptive Strike")