From Glossary of Meteorology
In U.S. weather observing practice, the greatest horizontal visibility that is equaled or surpassed throughout half of the horizon circle; it need not be a continuous half.
In the case of rapidly varying conditions, it is the average of the prevailing visibility while the observation is being taken. This value is entered in a surface weather observation. If the value is less than seven miles, it must be explained by reporting a type of weather or obstruction to vision.