a body of armed men acting under one commander: an army in the field, or any smaller force of soldiers sent out at his order.
This is the ordinary New Testament word for a force of fighting men, and its most striking feature is how freely it stretches. The same word covers the mounted host in Revelation that no one can count and the handful of Roman soldiers an officer in Jerusalem sends down a stairway to pull one prisoner out of a riot. Greek does not need a separate word for a great army and a small squad, because what the word actually names is not a size but a relationship: these are armed men who belong to someone and move when he says move.
That is why the word almost always comes with an owner attached. They are the king's troops, Herod's soldiers, the kings of the earth and their armies, the armies of heaven that ride behind the rider on the white horse. Whoever the men belong to is usually the point of the sentence, and the word carries that shape into all of its uses. The size of the force has to be read off the story; the word itself never tells you.
The noun is built on the verb "to serve as a soldier" (στρατεύω), and it names soldiers as a body rather than one at a time. Where a single man in the ranks is meant, the New Testament reaches for a different word (στρατιώτης); where the soldiers are pictured camped around a city, another one again (στρατόπεδον); and where the barracks they are quartered in is meant rather than the men, another still (παρεμβολή).