Lengthwise means along or in the direction of something's longest dimension. When you cut a carrot lengthwise, you slice it from end to end, creating long strips instead of round coins. When you fold a piece of paper lengthwise, you bring the two shorter edges together so the fold runs parallel to the longer sides.

Think of a swimming pool: swimming lengthwise means going from one end to the other along the longest stretch, while swimming widthwise (or across the width) means taking the shorter route from side to side. A carpenter might cut a board lengthwise to make it narrower, or you might arrange books lengthwise on a shelf so their spines face out.

The word helps us give clear directions about orientation. If you're arranging hot dogs in a baking dish, placing them lengthwise means lining them up so they run in the dish's longest direction. The opposite approach, crosswise or widthwise, would have them running the short way across.