Chametz is any food product derived of wheat, barley, rye, oats, spelt or its derivative.
During Passover there is a Biblical obligation to get rid of chametz: "You shall eat nothing leavened, in all your settlements you shall eat unleavened bread." (Ex. 12:20) Another place mentioned in the Torah implies communal responsibility for the destruction of chametz: "Throughout the seven days [of Passover] no leavened bread shall be found with you, and no leaven shall be found in all your territory." (Ex. 13:7)
Sources: Telushkin, Joseph. Biblical Literacy: The Most Important People, Events, and Ideas of the Hebrew Bible. NY: William Morrow and Co., 1997.