It may seem odd to see the question, but there are many people who do not know the answer. A quick search on the web will give you a definition like:
"a person who has served or is serving in the armed forces"
Under Title 38 of the US Code, the term "veteran" is defined as:
"The term “veteran” means a person who served in the active military, naval, or air service, and who was discharged or released therefrom under conditions other than dishonorable."