Definition - What does Rain Shadow mean?
A rain shadow describes the dry area on one side of a mountain or mountain range where less rain falls. This side of the mountain or mountain range is called the leeward side, while the side which has greater rainfall is called the windward side. A rain shadow is the result of the direction of wind and its effect on rainfall.
Safeopedia explains Rain Shadow
A rain shadow results as wind carries air full of moisture into mountains. This air is blocked by the mountains and cannot reach the other side. Precipitation or rain fall then falls on the side where wind is blowing from, but the area on the other side of the mountain is drier. This is a rain shadow.