flax: the fiber plant and the raw fiber combed from it, and the linen thread and cloth woven from it
One Hebrew word covers the whole life of a single material. It names the flax plant standing in the field and the pale fiber stripped from its stalks, and it names the finished product: linen thread, cord, belts, and garments. English splits this into two words, "flax" for the plant and its fiber and "linen" for the cloth, but Hebrew keeps them as one thing seen at different stages.
At the raw end it is the material a household works by hand. Rahab dried stalks of it on her roof to hide the spies (Joshua 2:6), the capable woman of Proverbs seeks out wool and flax and spins them (Proverbs 31:13), and the ropes on Samson's arms fell away like flax that has caught fire (Judges 15:14), since dry flax fiber flares up in an instant.
At the finished end it is cloth, and linen was the fabric of daily use and of the priesthood. The rules on mold in fabric weigh wool against linen (Leviticus 13), the two fibers the law forbids weaving into one garment (Deuteronomy 22:11), and the priests of the inner court are told to wear linen and nothing that would make them sweat (Ezekiel 44:17-18). Where the text wants the costly white cloth of the tabernacle it reaches instead for finer words, שֵׁשׁ and בַּד; this word stays closer to the plant and to ordinary linen.