I have never seen this verified anywhere, but I believe you are correct.
Since this project started out as 'yui-ext', and it was an extension to YUI, I have always assumed it was short for 'Extension'.
Of course it has snowballed into its own free standing library, so I don't think 'Extension' is really a fair word to use anymore. Still sounds nice though.
exactly. As much sense as "Dojo" or "MooTools" Or "Qooxdoo".
However it is not immediatly clear because this 3 letter word "Ext" would be mistaken for an abbreviation of some meaningful name.
I had asked this question because I have plans to purchase a licenced copy of Ext. But before doing that I wanted to find out what the official/legal name of the Ext JS foundation.