Loss of Separation
Loss of separation between aircraft occurs whenever specified separation minima are breached. Minimum separation standards for airspace are specified by ATS authorities, based on ICAO standards.
Types of Loss of Separation
Loss of separation between aircraft normally occurs when both the horizontal and the vertical minima are breached (the exception being composite separation) and may ultimately result in a mid air collision. A Level Bust is one scenario where a loss of separation occurs, leading potentially to a mid air collision. Loss of separation from notified airspace is dealt with under Airspace Infringement. Loss of separation from the ground is dealt with under Controlled Flight Into Terrain (CFIT). Loss of separation between aircraft on the ground is dealt with under Ground Operations and Runway Incursion.
Wake vortex, which may be a consequence of loss of separation between aircraft on the same track, is covered in the category: Wake Vortex Turbulence.
For further information read the lead article about Loss of Separation.