Flax is a linen fiber that is very akin to cotton, just more rugged at the expense of being less elastic. It's also notorious for creasing easily when used to make garments.
The resemblance of bundles of flax to golden blonde hair resulted in the coining of the term 'flaxen hair', and its properties have made it one of the most popular fibers for creating sheets. It also has a number of applications outside of the traditional domain of textiles, used in the creation of paper products and bank notes.
Flax has been used for at least tens of thousands of years, and was the primary material used by Sumerian priests for their robes at the height of their empire. Its ability to resist staining, dirt and wear has made it a very popular material for fine fabrics and cloth.