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As part of the three layer system, outer shell hiking pants are lightweight, usually made out of nylon or other very lightweight material, and are meant to be used as the last layer in the system. They are usually cut a little bigger to help them fit over the first two layers of clothing.
The fabric should be breathable, and it should be waterproof, as well. Strong nylon hiking pants fit the bill, because they are water and windproof, are lightweight, and breathable to take advantage of all three layers of clothing. In addition, outer shell hiking pants can serve as active gear for other sports when you're not hiking.