When I worked at Hewlett-Packard, the area in
between the first and second floors (or, for you europeans,
the ground and the first floors) was called the
"interstitial". There were a lot of wires and pipes and
deadly chemicals (beause we did some semiconductor fab onsite).
The interstitial was about 8-feet high, so I never really understood why they
didn't just make it an official floor.
I always thought that name was strange because
"interstitial" was an adjective that wasn't modifiying anything until
someone told me that "interstitial" was short for
"interstitial floor". I thought that that was even stranger since
"interstitial" is itself sesquipedalian. The obvious
abbreviation is IF which is not only shorter, but it's
also especially meaningful to fan's of Ender's Game.