EM also is electricity and magnetism. There are EM waves, EM equations, EM pulses...the electric and magnetic forces were the first to be unified at it happened so long ago that they are not listed as separate forces when the famous "four forces" are innumerated and often refered to as "electromagnetism" or, by those who are learning the field "electromaschism".
And that's the problem with two letter abbreviations: virtually all have been defined multiple times. With only 676 distinct combinations (if we tread lower and upper case equally and restrict ourselves to the Roman alphabet), the field of medicine alone, for example, could quickly run out.
Worse then that, every two letter combination corresponds to a curse word in a language which...oh never mind.