Mid-air Collision
Two aircraft collide in flight. Investigations centre on separation: who was responsible for it, what each crew could see, and what the collision-avoidance systems did.
A mid-air is a failure of separation, and separation is a shared responsibility between air traffic control, the crews' own lookout, and the traffic alert and collision avoidance system. A report will normally reconstruct all three.
Note the distinction the archive draws: a collision between two aircraft on a runway or taxiway is a ground collision, not a mid-air. Case files involving two or more aircraft are labelled "Multi-aircraft" unless the collision genuinely happened in flight.
Related terms
Airborne collision avoidance. It interrogates nearby transponders and, if a collision is likely, issues a coordinated climb or descend instruction to both crews.
An airworthy aircraft, under the crew's control, is flown into ground, water or an obstacle, usually because nobody on the flight deck knew the terrain was there.
Decisions and pressures outside the flight deck (dispatch, loading, scheduling, air traffic services, company culture) that shaped the outcome.