If something is corrugated, it has been shaped into regular waves or folds, usually to make it stronger without making it much heavier. The ridges stand up like a row of small hills. You see it in corrugated cardboard — the wavy paper in the middle of a box — and in corrugated iron or steel, the ridged metal sheets used on roofs and sheds. Those sheets are often dull and rusty in old buildings, so students sometimes think corrugated means “old metal,” but the word is only about the wavy shape. Plastic, paper, and even some fabrics can be corrugated too. It is an ordinary descriptive word, a little technical, not fancy. The verb behind it is corrugate, “to wrinkle or fold into ridges,” though the -ed form is the one you will hear most.