(aviation) Ceiling and Visibility are OK; specifically, (1) there are no clouds below 5000 feet above aerodrome level (AAL) or minimum sector altitude (whichever is higher) and no cumulonimbus or towering cumulus; (2) visibility is at least 10 kilometres (6 statute miles); and (3) no current or forecast significant weather such as precipitation, thunderstorms, shallow fog or low drifting snow.
Usage notes 
When appropriate, this term can be used in ATIS broadcasts in lieu of detailed weather information.
"CAVOK" may appear within METAR format weather report strings in place of the standard "Visibility Group (VVVVVSM)" as defined in Section 12.6.6 of "Federal Meteorological Handbook No. 1" (September 2005). However, it is to be noted that implementations of METAR in the USA may differ from that in other international locations, possibly to the extent that CAVOK is completely excluded in one implementation or the other.