Troop, n. — Webster enters it under “troop”
Etymology: [F. troupe, OF. trope, trupe, LL. troppus; of uncertain origin; cf. Icel. þorp a hamlet, village, G. dorf a village, dial. G. dorf a meeting. Norw. torp a little farm, a crowd, E. thorp. Cf. Troupe.]
1. A collection of people; a company; a number; a multitude. That which should accompany old age --As honor, love, obedience, troops of friends --I must not look to have. Shak.
2. Soldiers, collectively; an army; -- now generally used in the plural. Farewell the plumed troop, and the big wars. Shak. His troops moved to victory with the precision of machines. Macaulay.
3. (Mil.)
Specifically, a small body of cavalry, light horse, or dragoons, consisting usually of about sixty men, commanded by a captain; the unit of formation of cavalry, corresponding to the company in infantry. Formerly, also, a company of horse artillery; a battery.
3 more senses
4. A company of stageplayers; a troupe. W. Coxe.
5. (Mil.)
A particular roll of the drum; a quick march.
1 other Webster entry under this spelling
Troop, v. i. [imp. & p. p. Trooped; p. pr. & vb. n. Trooping.]
1. To move in numbers; to come or gather in crowds or troops. "Armies . . . troop to their standard." Milton.
2. To march on; to go forward in haste. Nor do I, as an enemy to peace, Troop in the throngs of military men. Shak.
Shades of meaning
Fernald has no entry of its own for troops; he lists it under army.
He lists it without comment; the entries above explain its neighbors.