A Reversal can refer to two things based on context: Either as a noun or as an adjective.
As a Noun
Refers to moves with invincibility at the very start and are often used as a way to counter the opponent's offense.
Common Examples of this are Sol's Volcanic Viper and Ragna's Inferno Divider.
As an Adjective
Refers to moves done immediately after leaving hitstun/blockstun.