Carrier-sense multiple access with collision avoidance (CSMA/CA) is a multi-part process to reduce collisions on a Wi-Fi, or 802.11, medium. It's similar to Carrier sense multiple access with collision detection (CSMA/CD) that is used for wired Ethernet connections. CSMA/CA is defined in the IEEE 802.11 standard.
The key difference is that on a physical medium such as a copper collisions can literally be detected, the medium of Wi-Fi is a little less robust as the carrier waves move through the air. If a collision occurs in Wi-Fi corruption may occur and 802.11 frames are dropped or they may never arrive at their intended recipient at all. It is CSMA/CA that works to reduce the occurrence of collisions by helping stations contend to access the medium.
Part of the process of CSMA/CA is to access the medium for other use in the frequency to which a station is tuned prior to transmission. Before a station even sets up to randomly count it's way down to send a wireless frame it listens for energy on the air and for other 802.11 frames. Stations following the IEEE 802.11 standard will use a network allocation vector timer (much better known as the NAV timer) to indicate how long they need to transmit their data. Other non-transmitting stations can use this timer, counting down to a point where the medium should be free. Before this a simpler medium check is used which detects energy on the medium (the air) - this is called Clear Channel Assessment.
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