briefcase

A briefcase is the flat case for carrying documents. Rectangular, hinged, with a handle on the long side.

It is named after briefs, the legal papers a barrister carries, which is where the shape comes from too. It has been the standard prop of office life for a century, though laptop bags and rucksacks have largely replaced it. Films use it as shorthand, since a briefcase handcuffed to a wrist means something important is inside. The word is one word, and an attaché case is the harder-sided cousin.