Thickness is how wide something is when you measure from one side straight through to the other. A paperback book might have a thickness of one inch, while a dictionary has a thickness of several inches. When you stack up pancakes, you're adding to the total thickness of the stack.

Thickness matters in surprising ways. The thickness of ice on a pond determines whether it's safe to skate on. The thickness of a rope affects how much weight it can hold. Pizza lovers debate whether thin crust or thick crust tastes better. Engineers calculate the thickness of bridge cables, knowing that thicker cables can support heavier loads.

In another sense, thickness can describe how dense or thick a liquid is. Honey has more thickness than water, which is why it pours slowly from the jar. When you make gravy, you increase its thickness by adding flour. Syrup, paint, and mud all have different levels of thickness that affect how they flow and behave.

People also use “thickness” when talking about fog, smoke, or crowds. When fog rolls in with such thickness that you can barely see your hand in front of your face, or when a crowd's thickness makes it hard to move through, the word captures that sense of density and how much something fills a space.