Knockdown is a term used in fighting games, including the Street Fighter series, to refer to a character's downed state that occurs when struck by some attacks.
There are two types of knockdown: "soft" or normal knockdown, and "hard" knockdown.
In the usual knockdown state, the default action is to get up normally after a second or two (known as wake up). Players can also quick stand upon knockdown by pressing buttons to recover faster.
If an opponent is stunned, the knockdown resulting from that attack becomes hard knockdown by default.
In Street Fighter IV, a glitch caused players to shift their position upon wake-up before they block if a direction was held. This made certain jump attacks after a Hard Knockdown unblockable unless the proper direction was pressed on the exact frame. In Ultra Street Fighter IV, the Delayed Wake-Up mechanic was introduced as a remedy, allowing players to slow their recovery and throw off foes.