A one-hit KO, commonly abbreviated as OHKO (also referred to as an Instant KO in Melee), is an attack in the Super Smash Bros. games with such high base knockback, that it KOs at 0%. However, moves that can OHKO are not necessarily guaranteed to do so, and may fail to OHKO under the following circumstances.
Knockback may be insufficient to KO at 0% if the attack had been landed previously (due to Stale-Move Negation).
Knockback may be insufficient to KO heavy characters at 0%.
The opponent is under an effect that makes them more resilient (such as being giant, metal, or in an event that reduces the knockback they sustain), or the player is weakened (such as being tiny or given a lower handicap).
The damage ratio is lower than standard, reducing knockback.
The general rule for a move to be considered a one-hit KO is being able to KO the majority of characters from the center of Final Destination at 0% in its normal trajectory without any outside factors affecting the knockback. Another common standard is to compare the move to the smash attack of the Home-Run Bat, the move intended to be the quintessential OHKO by the programmers.
Ike's Counter upon countering a ridiculously powerful attack; a hitbox that deals around 45% damage will result in an OHKO if countered. Marth's Counter can also potentially inflict an OHKO, though it must reflect an even more powerful move (a hitbox that deals around 50% damage).
If the Hothead is powered up significantly (e.g. through multiple reflections), it can deal well over 200% damage and OHKO.
If a Soccer Ball is launched at enough speed (after either being hit by an extremely powerful attack such as a Home Run bat swing, or being launched after receiving enough damage), it will OHKO anyone it slams into.