Veterans are individuals who have served in the military. The term 'pros' is a shortened version of the word 'professionals' and refers to veterans who have had extensive experience in the military. 'Old pros' are veterans who have been in the military for a long time and have a great deal of experience. 'Old hands' are veterans who have been in the military for many years and have a great deal of knowledge and expertise. Finally, 'old timers' are veterans who have been in the military for a very long time and have a great deal of wisdom and experience.