In a general sense, an ingredient is a substance which forms part of a mixture.
In cooking, recipes specify which ingredients are used to prepare a dish, and the term may also refer to a specific food item in relation to its use in different recipes.
National laws usually require prepared food products to display a list of ingredients and specifically require that certain additives be listed.
Law typically requires that ingredients be listed according to their relative weight within the product.
[1] From Middle French ingredient, from Latin ingredientem, present participle of ingredior ('to go or enter into or onto').