A mixture of wool and linen, forbidden by Biblical law.

Shatnez (or shaatnez / שַׁעַטְנֵז‎) is cloth containing both wool and linen, which Jewish law, derived from the Torah, prohibits wearing. The relevant biblical verses (Leviticus 19:19 and Deuteronomy 22:11) prohibit an individual from wearing wool and linen fabrics in one garment, the blending of different species of animals, and the planting together of different kinds of seeds (collectively known as kilayim).

Excerpted from Wikipedia