Lengthways means along the longest dimension or side of something. When you cut a carrot lengthways, you slice it from end to end rather than into round coins. When you fold a piece of paper lengthways, you bring the two long edges together so the fold runs the long way.

The word helps you describe direction and orientation. If you're arranging books on a shelf, you might place them lengthways so their spines face out. A carpenter cutting a board lengthways splits it along its length rather than across its width.

You'll also see the spelling lengthwise, which means exactly the same thing. Both words contrast with widthways or crosswise, which describe going across the shorter dimension. Think of it this way: a hot dog bun opens lengthways to hold the hot dog, but you'd cut a cucumber widthways to make round slices for a salad.