Veteran was a term used to describe a person who was experienced in a particular area, most often people with experience in the military and or combat.
Kira Nerys and members of the Shakaar Resistance Cell operated during the Cardassian Occupation of Bajor and participated in many combat actions. (DS9: "Shakaar", "The Darkness and the Light", "Ties of Blood and Water")
Li Nalas was a revered war hero of the Bajoran resistance durring the Cardassian Occupation of Bajor. (DS9: "The Homecoming")
Miles O'Brien as of 2372 had served in 235 combat encounters and had been decorated by Starfleet fifteen times and was considered an expert on starship combat. (DS9: "Rules of Engagement")