Kashrut (also kashruth or kashrus, כַּשְׁרוּת) is the set of Jewish dietary laws. Food in accord with halakha (Jewish law) is termed kosher in English, from the Ashkenazi pronunciation of the Hebrew term kashér (כָּשֵׁר), meaning "fit" (in this context, fit for consumption by Jews according to traditional Jewish law). Food that is not in accordance with Jewish law is called treif (Yiddish: טרײף or treyf, derived from Hebrew: טְרֵפָה trēfáh).
Many of the basic laws of kashrut are derived from the Torah's Books of Leviticus and Deuteronomy, with their details set down in the oral law that according to Jewish tradition was handed down by word of mouth down the generations from Moses and it was finally codified in the Mishnah which is the earliest portion of the Talmud. Later summaries of Halakhah such as the Shulhan Arukh, the Mishnah Berurah and other rabbinical authorities exist.
The Torah does not state reasons for most kashrut laws. Many varied reasons have been suggested, including philosophical, practical and hygienic. The Guide for the Perplexed, by Maimonides addresses this topic.
About one-sixth of American Jews maintain the kosher diet. Many Jews observe kashrut partially, by abstaining from pork or shellfish, or not drinking milk with a meat dish. Some keep kosher at home but will eat in a non-kosher restaurant.
Jews comprise only about 20% of the market for kosher food in the United States. A sizable non-Jewish segment of the population views kosher certification as an indication of wholesomeness. Strict vegetarians, Muslims, Hindus, and people with allergies to dairy foods, often consider the kosher-parve designation as an assurance that a food contains no animal-derived ingredients, including milk and all of its derivatives.
The word kosher has become a part of English slang, a colloquialism meaning proper, legitimate, genuine, fair, or acceptable.