The leaven in the bread that causes it to rise while being baked. It is used in a negative way throughout the Bible as a symbol of sin. The one exception to this would be in Matthew 13:33 where it is a symbol of growth and expansion.
The Rabbinical Assembly Pesah Guide was prepared for the Rabbinical Assembly Committee on Jewish Law and Standards by Rabbi Mayer Rabinowitz. It was accepted by the Committee on December 12, 1984. Parts have been amended to reflect more recent decisions of the Committee affecting the status of peanuts, peanut oil, certain cheeses and canned tuna.
The Torah prohibits the ownership of hametz (leaven) during Pesah. Therefore, we arrange for the sale of the hametz to a non-Jew. The transfer, mekhirat hametz, is accomplished by appointing an agent, usually the rabbi, to handle the sale. It is valid and legal transfer of ownership. At the end of the holiday, the agent arranges for the reversion of ownership of the now-permitted hametz. If ownership of the hametz was not transferred before the holiday, the use of this hametz is prohibited after the holiday as well (hametz she-avar alav ha-Pesah).
During the eight days of Pesah, hametz cannot lose its identity in an admixture. Therefore, the minutest amount of hametz renders the whole admixture hametz and its use on Pesah is prohibited. However, during the rest of the year, hametz follows the normal rules of admixture, i.e., it loses its identity in an admixture of one part hametz and sixty parts of non-hametz (batel beshishim). This affords us the opportunity to differentiate between foods purchased before and during Pesah.
Hametz (Chametz) is sometimes translated as bitter or sour. It is leaven and it causes dough to become puffed up. The sin of pride causes people to be puffed up.
Chametz is any food and drink made from wheat, barley, rye, oats, spelt or their derivatives.(1)
Prohibited foods include the following: leavened bread, cakes, biscuits, crackers, cereal, coffees containing cereal derivatives, wheat, barley, oats, spelt, rye, and all liquids containing ingredients or flavors made from grain alcohol.
Note: Matzah used all-year around is not for Passover use even though it contains no yeast. There are special "made for Passover" matzah's that are to be used specifically during Passover.
Read more about the Holiday Of Passover.
1). Chabad of Peoria 5766 Art Calendar.