Hilly describes land with many hills: a landscape that rolls up and down rather than staying flat. If you're biking through a hilly neighborhood, you'll constantly pedal upward, then coast down, then climb again. San Francisco is famous for being hilly, with steep streets that make your legs burn when you walk them.

The word tells you something important about a place. A hilly park might have great sledding spots in winter but be tiring to walk through. A hilly farm requires different equipment than a flat one. Runners training in hilly areas build stronger leg muscles than those who run on flat ground.

Notice that hilly doesn't mean mountainous: hills are gentler than mountains. Vermont is hilly; Colorado has actual mountains. When pioneers crossed America in covered wagons, hilly terrain slowed them down, but mountains could stop them completely until they found a pass through.

If you live somewhere hilly, you notice it every day: walking to school becomes a workout, rainwater runs downhill and collects in certain spots, and you can see farther from the tops of hills than your friends in flat places ever can.