What Is Chametz (Chometz)?
By Yehuda Shurpin

The Very Short Answer
Chametz (also spelled "hametz" or "chometz") is any food product made from wheat, barley, rye, oats or spelt that has come into contact with water and been allowed to ferment and “rise.”
In practice, just about anything made from these grains—is to be considered chametz. This includes cake, cookies, pasta, breads, and items that have chametz as an ingredient, like malt.
The Biblical Basis
Just before the nation of Israel left Egypt, G‑d commanded them to sacrifice the paschal lamb and then eat it with unleavened matzah and bitter herbs.2 G‑d then told them that they should replicate this feast every year on the anniversary of the Exodus: “It shall be for you a remembrance . . . seven days you shall eat matzah, and on the first day you should remove all se’or (sourdough, a leavening agent) from your homes. Anyone who eats chametz (leaven) from the first day to the seventh day shall be cut off from Israel.”
Getting Rid of Chametz
Long before Passover begins, we clean our homes, offices, and any other place that belongs to us to rid our homes of chametz. Although it’s praiseworthy to be stringent on Passover, keep in mind that dust isn’t chametz. The main purpose of cleaning and searching for chametz is to remove any chametz that one may come to inadvertently eat or derive benefit from during Passover. This obligation of getting rid of chametz does not extend to inedible chametz or tiny crumbs or particles of chametz that are soiled or spoiled. So the key areas to focus on are things that may come in contact with food, since we are forbidden to eat anything with even a trace of chametz.
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Excerpts from chabad.org