A network segment is a single run or 'span' of physical connectivity between the network interface of two computer devices. This physical layer connectivity could be a piece of copper or fiber-optic cable, an infrared or wi-fi link. A network is built by connecting together multiple network segments using a bridge, switch or router. Think of a network segment as the basic physical building-block that is used to build a complete network.
The term network segment is sometimes used to refer to a portion of the network between two networking devices such as between two switches or two routers. The term network segment is also used to describe a group of computer equipment organized as a network using the same range of addresses.