a roof having two slopes on all sides with the lower slope steeper than the upper one
About the Word:
The mansard roof gives extra space without requiring construction of an entire floor. The mansard was especially popular in Renaissance and Baroque France, but the term wasn't used in English until the 18th century; the word "mansard" honors the 17th century French architect François Mansart, who helped popularize (but did not create) the style.
("Mansard Roof" is also the first track on Vampire Weekend's 2008 debut album.)