Invincible is the state of being immune to taking any form of damage, you no longer feel pain from anything. Even if the painless state only lasts for a few moments, they would be rather intense and most likely void of all emotion.
Example: Drake: Is it just me or is he invincible? I've been throwing knives at him and he doesn't seem to mind it... It's so damn unbelievable!
Chris: Yeah... It's unbelievable. Quentin: Face it, Drake... I won't be flinching anytime soon!