BPU (basic performance upgrades) is a term borrowed from the Supra guys. For a turbo car, it's simply dp, ex, and some sort of boost controller. For an Integra, example, it'd be I/H/E. On the WRX, the uppipe is really part of BPU IMO.
Basically, it's a lot easier to say BPU than "Vishnu uppipe, godspeed down pipe, custom midpipe, tanabe catback, turbo XS manual boost controller" when someone asks what mods you have.