[nbashaw]

Quick nitpick: HAML is not a pre-processor for HTML, it is a templating language.

[liquidise]

Anything that is self-described as an "abstraction markup language" and must be compiled into the language it abstracts is really a pre-processor.

Regardless of intended use cases, i cant think of a better definition.

[cpleppert]

I disagree. A preprocessor doesn't 1) transform the entire document 2) doesn't fully parse the input document that is exactly why the c family on languages has such difficulty with modules: they are hacked on with a preprocessor and aren't part of the language.