We as airsofters throw it around really loosely.
In strict definition, none of us milsim. In fact, most of us look down upon milsim players.
(that team from mb which dare not be named) is milsim. They do 100% militarized ops, and airsoft is just a prop. They don't play airsoft, they play pretend war, and they do it within very strict guidelines.
What we refer to as milsim, is just staying as realistic as possible, within certain reasonable confines, yet maintaining the fun and atmosphere of airsoft.
Strict milsim is not fun for 95% of airsofters lol
The big thing for typical airsoft milsim is looking good, and having constant gameplay.
Milsim for us is 24 hour games with little or no rest period, no resets, dynamic gameplay, and usually some good tactics and actual planning thrown in. And realistic objectives, take and hold ground, rescue some dude, disarm a bomb.
Our general idea of milsim, is battlefield or COD but people actually go out when you shoot them lol