UK Aviation Definition
Definitions – ARPAS UK
Visual Line of Sight Operations (VLOS): Visual Line of Sight is termed as being the maximum distance that the flight crew is able to maintain separation and collision avoidance, under the prevailing atmospheric conditions, with the unaided eye (other than corrective lenses). For flights within Line of Sight, the pilot is required to employ the See – and – Avoid principle through continued observation of the aircraft, and the airspace around it, with respect to other aircraft and objects. Within the UK, Visual Line of Sight operations are normally accepted out to a maximum distance of 500 m horizontally, and 400 ft vertically, from the pilot.
US Definition
Visual Line of Sight. The pilot’s ability to see an aircraft from the ground
well enough to control it, without the use of artificial visual aids (aside from
glasses).