Clear Channel Assessment (CCA) is one of two carrier sense mechanisms in WLAN (or WiFi). It is defined in the IEEE 802.11-2007 standards as part of the Physical Medium Dependant (PMD) and Physical Layer Convergence Protocol (PLCP) layer. CCA involves two related functions - CCA-CS and CCA-ED.
Carrier Sense (CCA-CS): Ability of the receiver to detect and decode a WiFi preamble. From the PLCP header field, the time duration (in us) for which the medium will be occupied can be inferred and when such WiFi preamble is detected the CCA flag is held busy until the end of data transmission.
Energy Detect (CCA-ED): Ability of the receiver to detect non-WiFi energy in the operating channel and back off data transmission. The ED threshold is typically defined to be 20dB above the minimum Rx sensitivity of the PHY. If the in-band signal energy crosses this threshold, CCA is held busy until the medium energy is below the threshold.
For more info on CCA for the OFDM PHY (802.11a/g/n), you may want to read the CCA Sensitivity section 17.3.10.5 in the IEEE 802.11 standards document.