There's not really a hard limit for the percentage.
If an attacker has 47%, they might get lucky and maintain the longest chain for 6 blocks, which is enough for a devastating attack. On the other hand, they might have 53% for a while, and not manage a 6-block attack during that time.
Satoshi did a mathematical analysis of this in section 11 of his paper
So I think "51% attack" is a good name for this attack, even though it's not a complete explanation.