CAA's Airprox definition

"Some enquiries received by the Aviation Related Environmental Enquiries section come from members of the general public, who wish to report that they have witnessed a ‘near miss’. Whilst aircraft may appear to be operating at a similar altitude, it should be highlighted that even for the expert eye, accurately judging the height of 2 or more aircraft can be extremely difficult, especially if the higher aircraft is larger than the other. Nonetheless, the details of any such report are recorded and crosschecked against the MOR scheme database. However, it invariably proves to be the case that whilst the aircraft involved were on converging tracks, they were separated vertically, i.e. they were at different heights. Indeed, vertical separation is fundamental to air traffic systems worldwide, in the UK, at all levels and even in the worst weather conditions, air traffic controllers can cross aircraft 1000 feet above or below each other. In good weather, in areas where an air traffic control service is not mandatory, pilots can significantly reduce this 1000 feet vertical separation and still operate with complete safety on a ‘see and avoid’ basis."