Collision Domain

A collision domain is a logical area in a computer network where data packets can “collide” with one another, in particular in the Ethernet networking protocol. The more collisions in a network the less efficient it is.

A collision domain can be a single segment of Ethernet cable in shared-media Ethernet, or a single Ethernet hub in twisted-pair Ethernet, or even a whole network of hubs and repeaters.

If you are familiar with the layers of the OSI model you can easily determine what devices extend or break up collision domains.

In case you are not familiar with OSI model, here is what you need to know.

OSI Layer 1 Devices (hubs and repeaters) forward all data that is sent on the media and therefore extend collision domains.
OSI Layer 2 and Layer 3 Devices (switches and routers) break up collision domains.