For fellow techno-geeks...
Also represented by "D" for Directivity in some equations, to mean radiating into 1/2 space or 1/4 space etc. In other words, point sources radiate spherically, and if against the wall, only half of the sphere will will be able to propagate. In a corner, it can radiate into 1/4 space, and at the intersection of two walls and a ceiling (or floor), only 1/8 of the sphere expands into the room. This, of course, means that the radiation is directional, and not uniformly spherical in nature.
You can also use "Q" in the numerator for equations where "D" is in the denominator as you can't have a fraction as part of a fraction (D=1/4=Q=4). You could also just convert the fraction to a decimal (1/4=.25) and leave it where it is. An example of this would be the Critical Distance equation:
rc = ?R/16?D [rc = sq.root of R/16 pi D]
or
rc = ?QR/16? [rc = sq.root of QR/16 pi]
where R is the Room Constant
(edit: oops, math symbols didn't come through! 1st "?" is sq. root, 2nd "?" is pi)
OK, that's enough geekiness for now.