In telecommunications, a carrier sense multiple access with collision avoidance (CSMA/CA) is a network control protocol in which (a) a carrier sensing scheme is used, (b) a data station that intends to transmit sends a jam signal, (c) after waiting a sufficient time for all stations to receive the jam signal, the data station transmits a frame, and (d) while transmitting, if the data station detects a jam signal from another station, it stops transmitting for a random time and then tries again.
See also the similar Carrier sense multiple access with collision detection (CSMA/CD) protocol.
Source: from Federal Standard 1037C