It is a Geany ( http://www.geany.org/ ) plugin that high lights (highlights) the word that is currently selected in the editor. It works with geany-1.23.1. It also works with geany-1.24.1, and geany-1.25.
Great plugin! This is the feature I missed the most(from Notepad++). I hope this becomes an official plugin soon, or even better, integrated to source.