I'm on mobile, so can't easily find a link. But its basically the term landen used to describe using indentation to delimit block structure. There is a wiki page on it.
http://dl.acm.org/citation.cfm?doid=365230.365257 and gratis copies elsewhere; required reading since 1966. Of course "off-side" is a football term, somewhat significant in England later that year.
However, Python doesn't use Landin's off-side rule; colons are required, c.f. Haskell's usual syntax. Emacs works anyhow.
I prefer Python's use of colons to haskell's nothingness. Now...that means colons can't be used for types or cons, but it makes a lot of sense for python.
Ok, I figured as much. I've only ever seen 'significant whitespace' to describe this, but I guess that's not very clear either. I myself do like it as well, I just wish tabs were the default for indenting in editors and general best practices instead of spaces.
