What is the definition of sniper as noted in the biography, Marine Sniper?

A sniper is an infantry soldier or serviceman who specializes in shooting from concealed positions at longer ranges than normal. Snipers often use purpose-built rifles. Snipers are typically skilled in marksmanship, camouflage, observing, and field craft. Military snipers are routinely called upon to carry out infiltration, reconnaissance, and observation. During the period considered by the text, snipers worked in conjunction with a spotter.