A collision domain is a physical network segment where data packets can "collide" with one another for being sent on a shared medium, in particular in the Ethernet networking protocol. This is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it.
A group of Ethernet or Fast Ethernet devices in a CSMA LAN that are connected by repeaters and compete for access on the network. This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions. Collisions decrease network efficiency; if two devices transmit simultaneously, a collision occurs, and both devices must retransmit at a later time.