The military have many specific standards. For example, the radiOS they send out for soldiers to use in the field needs to be sturdier and more reliable than average, so there are specific parts that are able to maintain their values over many many conditions:
- wide temperature ranges
- wide humidity range
- certain levels of vibration and acceleration
- their values will be stable within certain ranges
- hardened against EMP so that nuclear explosions in the area do not destroy the parts
Shape, size, and parts are built for extreme conditions, so if something is "military grade," it means their parts are sturdy for this. There isn't regulation with this term, so this is often just used for marketing purposes.
