Final Turn (ファイナルターン Fainarutān) is an expression used by characters throughout the Cardfight!! Vanguard series, either occasionally or consistently. It was introduced by Toshiki Kai in the anime, although the origin of said expression is still unknown. While Kai is the one who made this phrase famous, he does not use it in every game that he wins. Kai's Final Turn call is usually accompanied by his respective soundtrack.
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Final Turn is the declaration of an assured victory, claiming that the user will win the fight in the same turn. It can be called either before starting the player's turn (Kai and most others) or after drawing their first card (Ren). With Aichi Sendou, it has become synonymous with his PSYqualia and later with his Shadow Paladin deck. The moment after failing to get a Final Turn can be considered embarrassing, as is evident by the reaction that the characters have shown regarding it.
It is noteworthy that in all scenarios where a Final Turn has failed by a PSYqualia user, the caller loses or the fight is ended in the next turn. With this in mind, it could suggest that PSYqualia only shows the user their final turn - not necessarily the final turn of the game.
It should also be noted that near the end of the anime's first season, Final Turn acquired rather negative connotations. Aichi used it in a cruel, derogatory manner to demean his opponents before finishing them off while under the influence of PSYqualia, and even Kai was implied to have begun using the phrase after Ren used it in a similar state. Kai mostly stopped using the phrase after he became less cold and single-minded about Vanguard, most importantly after Aichi fell under the influence of PSYqualia. It was used again sparsely during the second season, most importantly by Leon Soryu, who was possessed by PSYqualia. In season 3, it is repeatedly used mostly by "Яeverse" fighters.
At some time, Bushiroad has clarified that the use of this phrase can be considered as bad behavior, and even be a punishable offense on tournaments.