>Where can I find this information? what is the definition of an "active user"? thanks!
Good question, Jessie.  I looked at the page Mariya put together--https://meta.wikimedia.org/wiki/Research:Data--but I couldn't find it there.  
You can find it here: http://www.mediawiki.org/wiki/Analytics/Metric_definitions and "active user" should say "active editor".
D