According to the TCAS MOPS[1], Near Mid Air Collision (NMAC) occurs when two aircraft come within 100 feet vertically and 500 feet horizontally.

An AIRPROX which, unlike NMAC, has a formal but non-empirical ICAO definition, is commonly described as a Near Midair Collision (NMAC) in the USA but without necessarily applying the defintion quoted above. So whereas an NMAC is usually likely to also be an AIRPROX, an AIRPROX is not necessarily an NMAC.

See the main article AIRPROX

References

1. ^ Minimum Operational Performance Standards