Towelling is the cloth that towels are made of — thick, soft, and covered all over in tiny loops of thread that stand up from the surface. Those loops are why it works: they hold a lot of water, and they rub your skin dry. You can feel the difference with your eyes shut. A shirt is flat; towelling is furry and slightly bumpy. We use the word when we are talking about the material itself, not about one towel. So you say 'a towelling bathrobe' or 'a towelling headband' or 'these are made of towelling' — meaning: made of that towel cloth, even though the thing is not a towel. Bathrobes, baby bibs, beach cover-ups, the sweatbands tennis players wear on their wrists: all towelling. Spelling depends on where you are. British English writes towelling with two l's; American English writes toweling with one. Neither is wrong — they are the same word. Americans also often just say terry cloth or terry instead, and in the shops here in Taiwan you will see 毛巾布 on the label, which is exactly the same idea: towel cloth. One more use, a small one: towelling can also mean the act of drying something with a towel — 'a quick towelling down after swimming'. That one is less common, and mostly British.

毛巾布、毛巾料（起圈的那種厚棉布；台灣店家標籤常寫「毛巾布」）