What is "shatnez"

Shatnez (or shaatnez; Biblical Hebrew Šaʿatnez ) is cloth containing both wool and linen ( linsey-woolsey), which Jewish law, derived from the Torah, prohibits wearing. The relevant parts of the Torah ( Leviticus 19:19 and Deuteronomy 22:9–11) prohibit an individual from wearing wool and linen fabrics in one garment, the interbreeding of different species of animals, and the planting together of different kinds of seeds (collectively known as kilayim).