The term veteran is used to describe a person who is experienced in a particular area, most often people with experience in the military and or combat.
Notable Starfleet Veterans:
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")
Nog Participated in many campaigns in the Dominion War most noteworthily The Siege of AR-558.
Other Veterans:
Kira Nerys and members of The Shakaar Resistance Celloperated during the Cardassian Occupation of Bajor participated in many combat actions.
Li Nalas a revered war hero of the Bajoran resistance durring the Cardassian Occupation of Bajor. (DS9:"The Homecoming")