At which point does a strong MC become an Overpowered MC?

I'm writing an MHA fic where the main character (Izuku Midoriya) is a decent bit stronger than he was in canon. I've been told by a couple commenters that I should've tagged it as "Overpowered Midoriya Izuku" but I personally don't think I should do that, since to me, an OP character is so strong that they take away the stakes from the fight, while Izuku is constantly struggling in every fight he's been in so far.

So at which point does a strong character become overpowered? What even is overpowered in the first place?