Microscripting is where you "attach" code to a document - the code lives inside a document, rather than having a document live inside some code (like a CGI script).
Steve Ball, who has his own definition, most often uses the Tcl Web Server, although he also has a wrapper that allows him to use it from conventional CGI with SuiteSpot or comparable servers.
We think John Ousterhout first used the word.
I posted an example to comp.lang.tcl. Damian Cugley explains how the technique can powerfully improve the engineering of a site even when pages are served statically.
[Give other cross-references.]