An unblockable attack (or unblockable move) is an attack that cannot be defended against with a shield or a counterattack. Unblockable attacks tend to be exceptionally powerful and are often Final Smashes, wielded by bosses, or attacks from rare Pokémon and Assist Trophies. Most characters have at least one unblockable attack if Final Smashes and grabbing special attacks are counted; very few have one outside of these cases.
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.
All grabs are unblockable and so will not be included in the following list.
List of unblockable attacks by characters
List of unblockable attacks by items
List of unblockable attacks by enemies and bosses