Full form of AMRAAM

The full form of AMRAAM is 'Advanced Medium Range Air to Air Missile.'

A - Advanced
M - Medium
R. - Range
A - Air
A - Air
M - Missile

The AIM-120 Advanced Medium-Range Air-to-Air Missile, or AMRAAM  is a modern American beyond-visual-range air-to-air missile (BVRAAM) capable of all-weather day-and-night operations. 

Sources - Wikipedia