A herd is a large group of animals that live, eat, and move together. Cattle graze in herds across rangelands. Wild buffalo once traveled the American plains in massive herds numbering in the millions. Elephants live in tight family herds led by the oldest female. Sheep cluster together in herds, which is why shepherds use dogs to guide the whole group at once.

The animals in a herd stick together for protection and survival. When danger approaches, they can warn each other and confuse predators with their numbers. Young animals in the herd learn from older ones where to find food and water.

When used as a verb, to herd means to gather and guide animals as a group. A rancher herds cattle toward new pasture. Dogs bred for herding have an instinct to circle and guide groups of animals.

People sometimes use the word for human crowds too. If everyone in class suddenly rushes toward the door when the bell rings, moving together without thinking, they're acting like a herd. This gives us the phrase herd mentality, which describes when people follow what everyone else is doing instead of thinking for themselves. When you notice yourself doing something just because everyone else is, you're noticing herd mentality.