One Turn Kill (commonly abbreviated OTK) refers to reducing the opponent's whole life points (and any additional life they have gained) to 0 in a single turn.
While an OTK can be frustrating for the player receiving it and a fight where it happened could be considered "boring and one-side", OTKs often require large amounts of specific cards and resources and aren't unstoppable, if one fails the player who attempted it is often left with few to no resources left. Because of this, OTKs are often more a coincidence play that a player can achieve when a right hand and situation appears. Decks that focus on consistently pulling off OTKs however, do are closer to the negative view of OTKs as some players find it pointless to try to play against decks that will try and likely succeed to do that.
When an OTK is attempted, it is recommended to use cards that can nullify cards the opponent uses, as something as common as an attack nullify or damage reducer will often be enough to stop it.
There are 3 variations of OTKs
The most common one (and at the same time, the other variations will always also be this case). In this variation the player simply manages to reduce the opponent's life to 0 in the same turn, and it doesn't necessarily have to be the player's first turn. In theory, a player could stall for over 10 turns without dealing any damage despite having chances to do so and save the resources needed to perform an OTK, if they succeed, it is still considered a legitmate OTK.
First Turn Kill (FTK)
A First Turn Kill happens when the player wins in their first turn of the game, this is much harder than a standard OTK since they only have their opening hand to work with. It is also harder for the player who goes first as they only have 1 attack and don't draw a card that turn.
Tetsuya Kurodake nearly achieved a FTK in Episode 25 on the first turn of the fight but failed. Gao's OTK can also be considered a FTK since it happened on his first turn, even though it technically was the second turn of the fight.
One Hit Kill (OHK)
One Hit Kill is the hardest variation of OTKs, as it requires the player to reduce the opponent's whole life with a single hit (i.e. a 10+ critical direct attack or using an automatic victory card before dealing any damage). An additional difficulty with this variation is that if the opponent somehow pays any life before the 10+ damage (i.e. calling Street Racer, Eligos) is dealt, its already not a OHK.
Rouga Aragami achieved a OHK against Tetsuya in Episode 25 by increasing his item's critical to 10 which was the same Tetsuya's current life and succeeding to deal damage with it. It is noteworthy that the OHK nearly failed since attack the would've been negated had Rouga not had Fanged Dragon Declaration. It should also be noted that Rouga's OHK was also a FTK, thus meets all 3 variations at the same time, however he needed 9 gauge, 7 specific cards, and taking over 5 damage in his opponent's turn to pull it off, an example of OTK's inconsitency and high risks.