Hagalah is when water is boiled in a pot and a utensil is thrown in to kasher it. However, if a liquid other than water, say vinegar or oil, or a solution of water and another liquid is used. Can it be used to kasher a utensil?
It's a machlokes haposkim - see Rema, Orach Chaim 452:5, citing Orchos Chaim. The Ramban says it's no good, while the Rashba allows it. So the Rema says that it shouldn't be done, but bedieved (after the fact) it's okay.
(An article at OU Kosher mentions, among other things, the relevance of this to the kashering of chocolate-manufacturing equipment, where it may be difficult to use water.)