If a player dodges a monster's melee attack while wielding a long blade, there is a 1/3 chance they will riposte, letting them perform a reflexive attack against the monster (like a minotaur headbutt). Blocked attacks can't be riposted, and a monster can only be riposted once per their turn.
Riposte probability does not take into account weapon skill or weapon delay.
While long blades are normally the only weapons allowed to riposte, the Fencer's Gloves can allow any weapon to take advantage of riposting; if worn on a long blades user, it will increase the probability of riposting.
Long blade users should not expect ripostes to account for a significant portion of their damage, especially if they use a shield. It is certainly not worth sacrificing AC to maximize your EV bonus either.
Ripostes were introduced in 0.19, and additionally slightly decreased base damage for long blades across the board.