Kashrut (also kashruth or kashrus, כַּשְרוּת) refers to Jewish dietary laws. Food in accord with halakha (Jewish law) is termed kosher in English, from the Hebrew term kashér, meaning "fit" (in this context, fit for consumption by Jews according to traditional Jewish law). Jews may not consume non-kosher food (but there are no restrictions for non-dietary use, for example, injection of insulin of porcine origin). Food that is not in accord with Jewish law is called treif, (טרייף or treyf, Hebrew טְרֵפָה trēfáh). Treif meat is meat from a non-kosher animal or a kosher animal that has not been properly slaughtered according to Jewish law, but the term is applied by extension to all non-kosher food.

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 (the Mishnah and the Talmud) and codified by the Shulchan Aruch and later rabbinical authorities. The Torah does not explicitly state the reason for most kashrut laws, and many varied reasons have been offered for these laws, ranging from philosophical and ritualistic, to practical and hygienic.

By extension, the word kosher means legitimate, acceptable, permissible, genuine or authentic, in a broader sense.

Islam has a related but different system, named halal, and both systems have a comparable system of ritual slaughter (shechita in Judaism and Ḏabīḥah in Islam).