An inverted trough serving as a support and reflector for a flourescent lighting unit.
Usually a troffer is recessed into the ceiling.
Etymology: Blend of trough and coffer. Dictionaries list the first known use as 1942, but here's a 1940 ad on ebay (scroll about halfway down the page).
Most of the earliest references I've run across refer to Miller troffers, but I don't know if The Miller Company coined the word.