Refers to the material wool, typically from sheep. Used in compounds like ウールのセーター (wool sweater).
このセーターはウール100%です。
This sweater is 100% wool.
ウールのコートをクリーニングに出した。
I took my wool coat to the dry cleaner's.
This coat is 100% wool.
Wool shrinks easily, so I wash it by hand.
rock wool; mineral wool
steel wool