An area of reduced rainfall, usually on the lee side of a mountain range. Warm air can carry more water than cold air. For example, take a warm air mass cruising along the ground over Western Washington. It's probably pretty warm and carrying a lot of moisture from just having traveled across the Pacific. And kind of like a sponge it's going to dribble and spit water as it moves along. When it runs into the mountains, though, it's going to try to go up and over. When air goes up it cools off, and cool air can't hold as much moisture, so the sponge gets squeezed, so to speak, and it rains like crazy. By the time the air gets up and over the high country, most or all of the water is gone. So you can get the strange situation of a rain forest on one side of the mountains, and a few miles away in the rainshadow a desert.
a mountain range or other topographic feature that is crossed by prevailing winds, so that precipitation tends to fall more heavily on the upwind side, and the downwind side is relatively arid