"Prior enlisted" (colloquially "mustang") was a term used to describe military officers who had served as enlisted personnel before being commissioned. They may have attained a field commission, or have attended an officer training program or service academy.
Mustangs tended to be older and more experienced than peers-in-grade who became officers via a service academy.
Mustangs by service Edit
Kanril Eleya held the grade of sergeant in the Bajoran Militia when Space Arm was shut down in 2401. Her CO Colonel Karryn Retta got her a transfer to Starfleet and entry to Officer Candidate School. (Bait and Switch: "From Bajor to the Black, Part I")
By the early 25th century the Bajoran Militia required all officers to have served time as enlisted personnel. In an alternate timeline Kanril Eleya had attended officer school and had been commissioned, holding the rank of captain in 2409. (Bait and Switch: "The Road Not Taken")