Nodes are XenForo's equivalent of forums and categories in other forum software packages. A node can be one of several types, including a forum, and is organized into a tree structure. This allows you to define the structure of the site.
Nodes are more general in concept than forums, allowing you to include things in your "forum list" that normally wouldn't be possible.
Nodes are configured in the "applications" tab of the control panel.
Tough question.. as I roamed only international forums I rarely visited any local forums so my grammar on this is limited, but let me think..
Well, post is: הודעה
Thread is: שרשור
I would go with the word: צביר
The direct translation is "cluster" but personally it sounds fit to describe collection of forums and sub forums.
Just let him know it's not official, just idea of one person.