FranticFish - 7:28 am on Oct 14, 2011 (gmt 0)
I would have thought that a drop that large and that sudden could only mean either that...
(a) some algo tweak reassessed your site's worthiness for that term, regardless of its content, link profile etc, OR
(b) there was a significant (in Google's eyes) change to your content and link profile as regards that term
If you can pinpoint the exact date then reading threads here from shortly after that time could help if it's A. Getting an SEO experienced in working on mega-sites to do a 'site MOT' should help establish if B is a possibility.