A "near miss" in aviation terms would be anything close enough to potentially compromise safety if anything else went wrong. This tends to be taken very seriously – for example, every such reported incident in the UK is investigated and a report issued – even when the distances involved seem very large for a ‘near miss’. The UK investigative authority cites an example where the aircraft involved were 3 miles apart horizontally, and yet it was still handled as a safety issue ( http://www.airproxboard.org.uk/default.aspx?catid=423&pagety... ).

I expect the absolute distances considered critical for drone operations to be smaller, but still significant.

The level of scrutiny involved in aviation safety is very impressive. I’m not actually involved in the field myself, but find the human factors and systems engineering involved in keeping the skies safe to be very interesting. NTSB and CAA incident reports make fascinating reading!