A Reversal refers to moves with invincibility from the very start of the move, and are often used as a way to counter the opponent's offense. These moves are often used immediately after getting knocked down to interrupt the opponent's okizeme.
Common examples of this include Sol's Volcanic Viper and Ragna's Inferno Divider.
While these are the most common type of reversal, most characters often have a reversal option such as an invincible super attack.