An unblockable attack (also called unblockable move or unblockable) is an attack that cannot be defended against with a shield or a counterattack. Unblockable attacks tend to be exceptionally powerful and are often grabs, Final Smashes, wielded by bosses, or moves from rare Pokémon and Assist Trophies; there are very few moves that are unblockable outside of these cases. In Brawl, Smash 4, and Ultimate, most characters have at least one unblockable attack if grabs and Final Smashes are counted.
Unblockable attacks can still be avoided by intangibility, such as sidestepping and air dodging, as well as Bayonetta's Witch Time in most cases. True invincibility also protects against unblockable attacks, such as that granted by a Super Star.
It is possible for a shielding character to be hit by a blockable attack and an unblockable attack on the same frame. What happens in this situation depends on the game.
List of unblockable attacks by characters
List of unblockable attacks by items
List of unblockable attacks by enemies and bosses