a herd: a sizeable body of grazing animals kept together and driven about as one group
This is the plain Greek word for a herd. It is built on the ordinary verb meaning to lead or drive, so the picture behind it is a body of animals that is moved along together rather than a scattering of beasts that wander on their own. Greek writers used it of cattle, of horses, of goats, and of any livestock held as a single group.
In the New Testament it turns up in one episode only, told three times. Jesus crosses the lake into Gentile country and meets a man (two men, in Matthew's telling) tormented by unclean spirits, and the spirits beg to be sent into a herd of pigs feeding on the hillside nearby. Every occurrence of the word sits inside those few verses: four in Matthew, two in Mark, two in Luke. Mark alone puts a number on the animals, about two thousand.
Two things follow from that. The word names the group, not the animals in it: the pigs are a separate word (χοῖρος), and every verse has to say so. And the group acts as one body, which is exactly what the story needs it to do, since all three writers say that the whole herd rushed down the slope and drowned.
It is worth keeping this word apart from the Greek word for a shepherded flock (ποίμνη). That one carries the sheep and shepherd imagery the Gospels use for the people Jesus gathers. A herd in this sense is livestock held together and driven, and the New Testament never uses it of people.