Back in the day I was taught that a concert sound system should not be referred to as a "PA". A PA, my distinguised instructor stated, is a system used purely to address the public (think back to announcements made by your elementary school principal).
Problems tend to arise for me, however, when I describe my "sound system". People immediately start asking me to help them with their car stereos and home theaters. If I say "PA", folks generally know what I'm talking about.
Am I splitting hairs here, or should I stick to my pedantic guns?
-Matt