Dense describes something tightly packed together, with lots of matter squeezed into a small space. A dense forest has trees growing so close together that sunlight barely reaches the ground. A dense brownie feels heavy and compact, while a light, fluffy cake has more air mixed in.

In science, density measures how much mass fits into a given volume. A small rock feels heavier than a same-sized piece of foam because the rock is denser: its atoms are packed more tightly together. This is why dense objects sink while less dense objects float. A dense metal ball drops straight to the bottom of a pool, but a beach ball (full of air and not very dense) floats on top.

Dense can also describe writing or ideas that are hard to understand because they contain so much information packed together. When someone says a textbook chapter is dense, they mean it's full of complicated ideas that require careful reading and thinking.

People sometimes use dense as an insult, suggesting someone is slow to understand things, but this usage can be unkind.