Lutefisk is dried whitefish, usually cod, but sometimes ling or burbot, cured in lye. It is made from aged stockfish, or dried and salted cod. The fish takes a gelatinous texture after being rehydrated for days prior to eating.

A lye is an alkali metal hydroxide. Traditionally it was obtained by using rainwater to leach wood ashes, which are strongly alkaline and highly soluble in water, of their potassium hydroxide (KOH), producing lye water, a caustic basic solution. Then the lye water would either be used as such, as for curing olives before brining them, or evaporated of water to leave crystalline lye behind. "Lye" most accurately refers to sodium hydroxide (NaOH), but historically has been conflated to include other alkali materials, most notably potassium hydroxide.