A macro and/or template processor written in Tcl for Tcl, by William Duquette. See http://www.wjduquette.com/expand/index.html for its homepage.
This is a useful tool for conversion of tcl-based documents into other formats. The range of applications include
All of this falls under the broad category of Template and Macro processing.
BUT, with 8.5, expand becomes the closest thing Tcl has to a conventional keyword. [expand syntax ...]
Category Application | Category Package, a part of Tcllib (/textutil)