You're referring to "prevailing visibility" which is a measurement of the greatest distance visible throughout at least half of the horizon (as observed form a tower controller or a weather observation unit like AWOS or ASOS) and not necessarily continuously.
Runway Visual Range (RVR) is the distance (in feet or meters) over which a pilot of an aircraft (usually) at the runway threshold can see down the runway. RVR is measured by Transmissometer/s installed along the runway.
However, if the visibility is less than the minimum visibility for the lowest approach (usually ILS) of the respective runway, it is not recommended to commence takeoff.