ICAO Annex 3 Chapter 4 para 4.13.2. states:
When the following conditions obtain simultaneously at the time of
observation:
visibility,
10km or more;
no
cloud below 1,500m (5,000ft) or below the highest minimum sector altitude,
whichever is greater, and no cumulonimbus;
no
weather phenomena of significance to aviation;
information on visibility, runway visual range, present weather and cloud amount,
type and height shall be replaced in all meteorological reports by the term
“CAVOK”.