What is a chimney in climbing?

Think about it as the shape of a chimney on a house. To "Chimney" one must climb between opposing rock faces, with the back and hands against one face, and the feet against the other face, or alternating between both. The chimney doesn't have to be contained on all sides, like a tunnel, or house chimney, so long as it has 2 sides to push agianst as you climb.