Source : Free On-Line Dictionary of Computing

off-side rule
     
        A lexical convention due to Landin, allowing the scope of
        declarations in a program to be expressed by indentation.  Any
        non-whitespace token to the left of the first such token on
        the previous line is taken to be the start of a new
        declaration.  Used in, for example, Miranda and Haskell.
     
        [P.J. Landin "The Next 700 Programming Languages", CACM vol 9
        pp157-165, March 1966]