we all know what it means - someone who has become a millionaire in USD from investing in bitcoin.
if the community decide that is what the term means then that is what it means and that how terms become real.
How long until this becomes a poll in Bitcoin Discussion?
I guess there's no need for a poll. The factual definition is: owner of at least one million bitcoins, but the usage that many people are familiar with is that a person has at least one million dollars/pounds/euros (typically) worth of bitcoin.
As somebody else pointed out, the flaw with the latter usage is that you could easily be considered a Bitcoin millionaire in rules, or yen etc haha.