Mostly this refers to a type of cotton thread. It comes in skeins of 6 strands loosely twisted together, and with a color-coded number. It is normally sewn in cross stitch using only two or three strands in a needle. There is also silk, rayon, metallic and nylon floss. Floss is used as the main ingredient in sewing a cross stitch designs. Most cross stitch designs are sewn with only floss.