To me, a spike is when my levels temporarily rise above my target range and then return to in range within a short time of about 30 minutes.
I have seen some people refer to the rise as the spike but in my mind this is not "spike shaped".
A "plateau" (a rise that remains and does not come down) is not a spike in my mind.
So, it is less about how high it goes but more about how long it stays there.
That said, I would not consider 8.5 to be a spike but I would correct it if it is a stubborn 8.5 that is not coming down when I have no insulin on board.