is a hydroxide made by leaching
wood ashes with water. It is used to make soap
Lye is a hydroxide obtained by leaching ashes from hardwood with water. It modern usage, lye most commonly refers to sodium hydroxide, whereas historically it referred to potassium hydroxide.
Lye is used with oil or fat to make soap, with sodium hydroxide used for bar soaps and potassium hydroxide used for liquid soaps.