So far there hasn't been a consensus on what "eos" should stand for. There have been ideas thrown around like "ethereum on steroids" and "ethereum on steem". @dantheman has not yet himself officially stated what it stands for.
I propose that eos stands for "(e) Operating System" much like email stands for electronic mail. In this case an operating system is already electronic so to speak but the e could stand for enhanced, energized, enabled, everyone's, elevated...
After watching the announcement here:
You will see that eos is indeed the blockhain operating system and it's going to change the world!
So what do you think the "e" in eos should stand for?