The strict rabbinical interpretation of chametz is any food prepared from five specific grains (wheat, spelt, oats, barley, rye) that has been allowed to leaven (ferment). The rabbis determined that leavening takes place when the grains have been in contact with water for more than 18 minutes. In practice, chametz refers to all grain products like cereals, flours, mixes, crackers, cookies, cakes, breads, rolls, etc. Additionally, any food that contains any of the five grains in any amount is considered chametz. The only grain products allowable during Pesach are those that have been stricly controlled and supervised by a Mashgiach ( kashruth supervisor) and are accordingly certified as kosher for Passover. See the explanation of kitniot below for a further interpretation of chametz.
Leavening Yeast, baking powder, mixes with a leavening agent, bread and bread crumbs. Literally sour