Carrier-sense multiple access

Carrier Sense Multiple Access (CSMA) is a Media Access Control (MAC) protocol. When transmitting on a medium that is shared by many transmitters, the transmitter first checks to see that nothing else is already transmitting. Examples of shared transmission medium include an electrical bus, or part of the electromagnetic spectrum.
"Carrier Sense" means that a transmitter uses feedback from a receiver to know if the medium is already in use before trying to send. That is, it tries to find a signal from another station before trying to transmit. If a signal is found, the station waits for that transmission to end before starting its own transmission.
"Multiple Access" means that many stations send and receive on the medium. Transmissions are usually received by all other stations using the medium.