The International Civil Aviation Organization classifies occurrences into five categories depending on their severity.
An incident involving circumstances indicating that an accident nearly occurred. Note: The difference between an accident and a serious incident lies only in the result.
An incident associated with the operation of an aircraft, in which safety of aircraft may have been compromised, having led to a near collision between aircraft, with ground or obstacles (i.e., safety margins not respected which is not the result of an ATC instruction).
An incident involving circumstances indicating that an accident, a serious or major incident could have occurred, if the risk had not been managed within safety margins, or if another aircraft had been in the vicinity.
Insufficient information was available to determine the risk involved or inconclusive or conflicting evidence precluded such determination.
An incident which has no safety significance.