A roller-skate is a special boot with wheels fixed under it, used for skating on hard ground, not on ice. The old, textbook kind has four wheels: two at the front and two at the back, like a little car under each foot. You strap them on, push off, and glide. The word names the boot, and you can also roller-skate as a verb. Ice skates are a different thing (a blade, and you need ice). The newer boots with wheels in one straight line are usually called inline skates; though actually many families still say roller-skates for both, which is loose but very common.

旱冰鞋、四輪溜冰鞋（不是直排輪）