A soft, white, fibrous material that comes from plants of the mallow family (genus Gossypium). These fibers surround the seeds in its round seed pod (called a boll). People weave cotton into thread and cloth (Isaiah 19:9).
Cotton plants or shrubs grow in warm climates. They are valuable both for the soft white fiber attached to their seeds and for the oil that can be extracted from these seeds.
The "white linen" in Esther 1:6 is almost certainly referring to Levant cotton (Gossypium herbaceum). This type of cotton has been grown since ancient times in the Far East. Alexander the Great brought cotton back to the western world from India.
The Jewish people probably became familiar with cotton during the time they were held captive in Persia under King Ahasuerus.