With multi-access link technologies such as Ethernet, there is a possibility that two or more hosts will send frames into the shared channel at the same time. When this happens, the result is called a collision.
Collisions can be detected on electrical media via the increased voltage that occurs when two messages overlap. This type of collision detection is at the heart of Ethernet's CSMA/CD approach. An alternative strategy is to use polling or some other mechanism to avoid collisions altogether. An example of such a non-collisional approach is WiFi's CSMA/CA.