Oh dear, I expect my Air Force service is going to show up here.
I wasn't a pilot, but envelope is a word used by them, in the context of defining and understanding the performance envelope of each aircraft type (take off and landing speed, stall characteristics etc.) so that it can safely be flown without exceeding its envelope - it's more than a little embarrassing to have your plane fall out of the sky because you strayed outside its envelope. In other words, envelope is another word (to my mind) for limits. Limits are things that occur in any relationship - perhaps especially kinky ones.
Limits are things that can be err, interesting to play around the edges of, and perhaps even to push against - some of the hottest scenes we ever did were those that explored your limits. There are hard limits that are very unlikely ever to be broken, soft limits that are perhaps scary and interesting at the same time and will be explored. Limits can also change over time, especially as a relationship matures. You were new to BDSM when we got together, and had many limits in place because you were nervous (but excited as all hell) and we needed to create a safe-ish space for you to explore within. Many of those limits went away over time, I seem to remember.
Now of course, with your illness, some of the limits we play within are even tighter than those very early ones, but we both do what we can.
So our envelope has changed many times during the years we've been together, but it's pretty fucking amazing for all that!
Previous posts from So I asked SilverHubby here.