The Collaborative International Dictionary of English v.0.48:

Lean-to \Lean"-to`\, a. (Arch.)
   Having only one slope or pitch; -- said of a roof.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lean-to \Lean"-to`\, n.
   1. (Arch.) 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.
      [1913 Webster]

            The outer circuit was covered as a lean-to, all
            round this inner apartment.           --De Foe.
      [1913 Webster]

   2. A crude, usually temporary shelter comprising a lean-to
      roof braced against any convenient support, as a wall, a
      tree or a pole. The roof may extend all the way to the
      ground.
      [PJC]

WordNet (r) 3.0 (2006):

lean-to
    n 1: rough shelter whose roof has only one slope