The Free On-line Dictionary of Computing (30 December 2018):

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]