Source : Webster's Revised Unabridged Dictionary (1913)

Lean-to \Lean"-to`\, a. (Arch.)
   Having only one slope or pitch; -- said of a roof. -- n. A
   shed or slight building placed against the wall of a larger
   structure and having a single-pitched roof; -- called also
   {penthouse}, and {to-fall}.

Source : WordNet®

lean-to
     n : rough shelter whose roof has only one slope