The answer to the crossword for the term 'ingredient in soap' is LYE, which is a three-letter word. LYE is an alkaline solution that is used as an ingredient in soap and other cleaning products. It is made from either sodium hydroxide or potassium hydroxide, both of which are strong bases. LYE is also known as caustic soda or sodium hydrate. It is used to break down fats and oils, making them easier to wash away.