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. Many commercial products contain secret ingredients purported to make them better than competing products. In the pharmaceutical industry, an active ingredient is the ingredient in a formulation which invokes biological activity.

National laws usually require prepared food products to display a list of ingredients and specifically require that certain additives be listed. In the European Union, food labeling regulations also mandate that allergens such as nuts, milk, and gluten be clearly emphasized within the ingredient list to protect consumers. Furthermore, regulations enforced by agencies such as the United States Food and Drug Administration (FDA) mandate that ingredients need to be listed in descending order of predominance by weight. This structural requirement ensures that the ingredients present in the greatest quantity appear first, allowing consumers to accurately assess the primary composition of a product.