A colony of single-celled organisms is known as a colonial organism. Colonial organisms were probably the first step towards multicellular organisms via natural selection. The difference between a multicellular organism and a colonial organism is that individual organisms from a colony can, if separated, survive on their own, while cells from a multicellular lifeform (e.g., cells from a brain) cannot. Volvox is an example for the border between these two states.
A bacterial colony (or colony of other microorganisms) is defined as a cluster of organisms growing on the surface of or within a solid medium, usually cultured from a single cell. Because all organisms within the colony descend from a single ancestor, they are genetically identical (except for mutations which occur at a low, unavoidable frequency), thus this technique can be used to isolate genetically pure strains from a genetically mixed population.