Densely means packed tightly together with little empty space in between. A densely populated city like Tokyo or New York has millions of people living close together in a relatively small area, with buildings crowding next to each other and streets filled with activity. A densely wooded forest has trees growing so close that their branches tangle together and sunlight barely reaches the ground.

The word describes concentration and closeness. Text printed densely on a page has small letters squeezed together with narrow margins, making it harder to read than text with more breathing room. A densely woven fabric has threads packed so tightly that water can't seep through. Scientists might describe a material as densely packed when its molecules are compressed together with minimal space between them.

The opposite is sparsely, which describes things spread far apart. A sparsely populated area like rural Montana might have just a few neighbors within miles, while a densely populated neighborhood might have dozens of families living on a single block. When you pack your backpack densely for a camping trip, you're fitting as much as possible into the available space, leaving no room unused.