ac·cou·ter·ment or ac·cou·tre·ment
As well as defining a setting, accoutrements are the tools of the player characters to interface with the setting. A Jedi uses his Mind-Trick to confuse the Stormtrooper - the same operationally as a Plumber uses his wrench to fix the sink. The player - through the character - uses the accoutrement to perform an operation on the environment of the setting. A naval captain can use his uniform to gain admittance to a party to which he was not invited. It's the use of the item as a tool which makes it an accoutrement.
Settings need accoutrements to work, but accoutrements are defined by the system - even if the definition of musket is the equivalent of "works like a musket" as in some very light systems.
Too often, accoutrements are lumped in with color. The only accoutrements which consistently tend to be well-defined in RPGs are weapons and armor, but with a range of well defined and operational accoutrements, one doesn't need an actual setting. The accoutrements do the work for you by implying the setting. Wearing clothes is color. Wearing the right clothes for the right situation is using an accoutrement.
Mike Crow once wrote an article about accoutrements in which he listed three degrees of accoutrement. I can't find it now - the blog has apparently died - but these levels were something like "usual" - the accoutrement is standard and easily found, like a uniform; "limited" - the accoutrement is not easy to find, and more powerful in it's effects, like a grav-bike; and "defining" where this item alone can define a setting - like a Sandworm. Sandworms are monsters, but since the Fremen use them for transportation in Dune, they are actually accoutrements. Neat idea!