Atzeret (עצרת) comes from the root עצר which means (nifal, as a verb) to stop, be delayed or to be arrested.
But I also heard that R.S.Hirsch explains it means 'to collect, to store or to gather' as in the root עוצר
But I can't find the source for this, so I would like to ask if anyone knows the right source or any other commentary which explains it as such.