An ingredient is a part that forms part of a mixture (in a general sense). For example, in cooking, recipes specify which ingredients are used to prepare a specific dish. Many commercial products contain a secret ingredient that is purported to make them better than competing products. In the health industry, an active ingredient is that part of a recipe that yields the effect required by the buyer.
National laws usually require prepared food products to show a list of ingredients, and require that certain "bad things" be listed.
In most developed countries, the law requires that ingredients be listed according to their weight in the product. If an ingredient itself consists of more than one ingredient (such as the cookie pieces which are a part of "cookies and cream" flavor ice cream), then that ingredient is listed by what percentage of the total product it occupies, with its own ingredients showed next to it in brackets.