WYSIWYG means "What You See Is What You Get". The term "WYSIWYG editor" is used to refer to software in which users edit text (or other types of content) graphically with an user interface that presents content in a form that is close to the one achieved as a final product (e.g. when printed).
In contrast, non-WYSIWYG text editing requires the use of a markup language, such as TWiki Markup Language (TML
or the TeX typesetting software popular in acadaemia.
The latest production release of TWiki ships with a TinyMCEPlugin