One hit knockout

A One Hit Knockout (OHKO) is a term used for attacks that, if successful, knocks the other player instantly. This is widely discouraged throughout the Mugen community.

Examples
The Dopefish's Slowest Bite.

Mortal Kombat characters fatalities.

God Orochi's Celestial Star

Most of Hastur's Supers

Apollo Mizuchi's Shattering Glass move

Pingu's Super Scary Attack & Pin-Goku-Satsu.

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Rare Akuma's Shun Goku Satsu are examples of One Hit KO's.

"Untrue" One Hit Knockouts
Certain attacks aren't true One Hit Knockouts, but cannot be survived through normal means nonetheless. An example is Metool's Giant Metool drop, AKA, and 0WN3D, which has a 9000 attack power. A possible reference to the "Over 9000" fad. Most overpowered Dragon Ball Z characters have energy waves that deal damage at a constant rate instead of one extremely strong blow.