A troop is a squad or team of soldiers. Your great grandfather might claim that his was the first army troop to march into France during World War II.
Though it's correct to use the noun troop in the singular form, it's more common to see its plural, troops. You can also refer to a group of Girl or Boy Scouts this way: "My troop won a prize for selling the most Girl Scout cookies this year." To troop is also to march, so a military troop might troop down the street in the Veteran's Day parade. The root word is the Old French trope, "band of people."