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Leavened foods that are forbidden on the Jewish holiday of Passover

Chametz (also chometz, , ḥameṣ, ḥameç and other spellings transliterated from ; ) are foods with leavening agents that are forbidden to Jews on the holiday of Passover. Chametz is a product that is both made from one of the five species of grain and has been combined with water and left to stand raw for longer than eighteen minutes (according to most opinions) and becomes leavened. This law appears several times in the Torah; according to halakha (Jewish law), Jews may not own, eat or benefit from chametz during Passover. The penalty for eating chametz on Passover is the divine punishment of kareth (cutting off).

The adjective chametz is derived from the common Semitic root Ḥ M Ṣ, relating to bread, leavening, and baking. The related noun chimutz is the process of leavening or fermenting. It is cognate to the Aramaic חמע, "to ferment, leaven" and the Arabic حَمْض ḥamḍ, "acid", حَمُضَ ḥamuḍa "to be sour", "to become acidic", "to acidify". This root relates to acidity and sourness in Hebrew as well, as the word chometz חומץ means vinegar, and the word chamootz חמוץ means sour.

Leavening agents, such as yeast or baking soda, are not themselves chametz. Rather, it is the fermented grains. Thus yeast may be used in making wine. Similarly, baking soda may be used in Passover baked goods made with matzoh meal and in matzoh balls. Since the matzoh meal used in those foods is already baked, the grain will not ferment. Whether a chemical leavener such as baking soda may be used with flour in making egg matzoh is disputed among contemporary Sephardic authorities. In accordance with those who permit it, cookies made with Passover flour, wine and a chemical leavener (the absence of water would make them similar to egg matzoh under the chametz rules) are marketed in Israel under the name "wine cookies" to Sephardim and others who eat egg matzoh on Passover.

The Torah specifies the punishment of kareth, one of the highest levels of punishment in Jewish tradition, for eating chametz on Passover (Exodus 12:15 (HE)). During Passover, eating chametz is prohibited no matter how small a proportion it is in a mixture although the usual rule is that if less than 1/60 of a mixture is not kosher, the mixture is permitted. If the dilution happened before Pesach, the usual 1/60 rule applies; Ashkenazi Jews apply this leniency only if the mixture is liquid. Also, hana'ah (any benefit, such as selling) from some forms of non kosher food is permitted, but no form of benefit may be derived from chametz during Passover. Mixtures consisting of less than 50% chametz that are not usually consumed by people (such as medicine or pet food—even if perfectly edible) may be owned and used on Passover but may not be eaten.

In addition to the Biblical prohibition of owning chametz, there is also a positive commandment to remove it from one's possession. There are three traditional methods of removing chametz:
Bi'ur: destroying one's chametz. All appropriate methods of destruction, like burning, are included in this category. On the night preceding the 14th of Nisan, a formal bedikat chametz (search for chametz) is conducted by candlelight. The chametz found in this search is burned the next morning, in a formal bi'ur ceremony. Bittul: nullifying one's chametz. On the night and again on the morning of the 14th of Nissan, at the formal bedikah and bi'ur respectively, the head of the household recites an Aramaic statement nullifying all chametz remaining in the family's possession. The statements conclude that the chametz "shall be nullified and considered ownerless as the dust of the earth." Bittul must be done before the prohibition of chametz takes effect; once five twelfths of the day have passed on Passover eve, bittul is no longer an effective means of removal, and any chametz that one discovers must be destroyed. Mechirah: selling one's chametz. Until five twelfths of the way through Passover Eve one may sell or give one's chametz to a non Jew, and it is no longer one's responsibility. One who keeps the sold chametz in a household must seal it away so that it will not be visible during the holiday. After the holiday, the non Jew generally sells the chametz back to the original owners via the agent; nevertheless, he is under no obligation to do so. It is considered best to use both bi'ur and bittul to remove one's chametz even though either of these two methods is enough to fulfill one's biblical requirement to destroy it. Mechirah, which averts the prohibition of ownership, is an alternative to destruction.

In many Jewish communities, the rabbi signs a contract with each congregant, assigning the rabbi as an agent to sell their chametz. The practice is convenient for the congregation and ensures that the sale is binding by both Jewish and local law. For chametz owned by the State of Israel, which includes its state companies, the prison service and the country's stock of emergency supplies, the Chief Rabbinate act as agent; during the 2000s, the Rabbinate sold its chametz to Jaaber Hussein, a hotel manager residing in Abu Ghosh, who puts down a deposit of 20,000 shekels for chametz worth an estimated $150 million.

According to halakha (Jewish law), if chametz is found during Shabbat or Yom Tov, it must be covered over until Chol HaMoed, when it can be burned. Chametz found during Chol HaMoed (except on Shabbat) should be burned immediately. After the holiday, there is a special law known as chametz she'avar alav haPesach (chametz that was owned by a Jew during Pesach). Such chametz must be burned, since no benefit is allowed to be derived from it, not even by selling it to a non Jew. Chametz she'avar alav haPesach may not be eaten by Jews after Pesach. If a store owned by a Jew is known not to have sold its chametz, a Jew may not buy any from that store until enough time has passed in which it can be assumed that the inventory has changed over since Pesach.

Because of the Torah's severity regarding the prohibition of chametz, many communities have adopted stringencies not biblically required as safeguards from inadvertent transgression.

Among Ashkenazi Jews, the custom during Passover is to refrain not only from products of the five grains but also kitniyot (lit. small things), which refers to other grains or legumes. Traditions of what is considered kitniyot vary from community to community but generally include rice, corn, lentils, and beans. Many include peanuts as well. The custom of kitniyot is observed by Ashkenazi Jews. Some Sephardi Jews from Spain and North Africa (for example, Moroccan Jews) have different restrictions, such as avoiding rice during Pesach. In recent years, there is some movement among Conservative as well as some Orthodox Ashkenazi Jews to cease to observe the tradition of kitniyot.

Matzo is generally made from flour and water. If made from flour and a different liquid, such as fruit juice or eggs, it is not considered chametz. Ashkenazi custom is generally to avoid such products, in case some water was mixed into the liquid, which could cause the mixture to become chametz. This product is known as "egg matzo" or "enriched matzo".

At Passover, some Hasidic Jews will not eat matzo that has become wet, including matzo balls and other matzo meal products although it cannot become chametz. Such products are called gebrochts (Yiddish: broken), referring to the broken or ground matzo used for baking or cooking. Instead of matzo meal, they use potato starch in cakes and other dishes. The Hebrew term for gebrochts is matzah sh'ruyah (, soaked matzo), but outside Israel, the Yiddish name is usually the one that is used.