In biology, a colony (from Latin colonia) means several individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defences, the ability to attack bigger prey etc. Some insects (ants, for example) live only in colonies.
Colonies were probably the first step towards multicellular organisms during evolution. The difference between a multicellular organism and a colony is that individual organisms from a colony can, if separated, survive on their own, while cells from a multicellular lifeform (e.g., liver cells) cannot. Volvox is an example for the border between these two states.