MSA

MSA - minimum sector altitude or minimum safe altitude.
msl - mean sea level
MTBF - mean time between failures. Also MTTR, mean time to repair ...

Minimum safe altitude (MSA). The minimum altitude depicted on approach charts which provides at least 1,000 feet of obstacle clearance for emergency use within a specified distance from the listed navigation facility.

See also: Aviation, Service, Aircraft, Military operations area, Middle marker