A collision domain is a physical network segment where data packets can collide with one another when being sent on a shared medium, in particular, when using the Ethernet networking protocol. A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. Collisions are resolved using using Carrier Sense Multiple Access (CSMA/CD) or a variant thereof in which the competing packets are discarded and re-sent one at a time. This becomes a source of inefficiency in 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. Collision domains are also found in wireless networking such as Wi-Fi. 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. Because only one device may be transmitting at any one time, total network bandwidth is shared among all devices. Collisions also decrease network efficiency on a collision domain; if two devices transmit simultaneously, a collision occurs, and both devices must retransmit at a later time.

To relieve the network of collision domains, it is recommended to use a network switch which increases the number of collision domains, but decreases each collision domain's size. This is because each port on a switch is its own collision domain.