Would be something in an embedded context.

Mission-critical is actually a pretty common term and not really connected to the military.

I don't disagree that it's used outside of defense. It's just used very very commonly in defense. Just harmless speculation ;).

Big bucketloads of business terms and concepts were stolen from the military.

Tactical advantages, strategic initiatives, anyone? And what about the ranking system in your average BigCo? They guys aren't called Sergeant or General but in effect it's the same.