The term, little language, comes from Jon Bentley's column in Communications of the ACM 29:8 (August 1986), pp. 711-721. He points out that much of what we do as programmers can be thought of as creating "little languages" that are suited to particular small tasks.
For a very good example see Solving cryptarithms, especially the examples at end! To prevent over-deep brace nesting, a custom language for such riddles is designed and put to fascinating use.
The Tcl language is itself "little:" its formal syntax is described completely in eleven short paragraphs on the Tcl manual page. Most of its power comes from the fact that it is easy to embed other little languages within it. Obvious examples include:
All of these are comparatively small and self-contained, and do not have far-ranging effects on the rest of the Tcl system. In this way, Tcl can be understood in pieces; it's not necessary to know the whole language in order to be able to program in it.
Some little languages that are not part of the Tcl/Tk core are:
Little Language http://c2.com/cgi/wiki?LittleLanguage
for other little languages.