Terms Used and What Exactly We Mean by Them

Ceteris Paribus: This means essentially, "everything else held constant." We presume this to be the case unless explicitly stated otherwise. We shall not use the actual term on the site, but it is to be understood to be the default case.
Empirical: From the evidence of one's senses. Things that have been directly observed and/or measured.
NA: Most of the time this will be used to denote North America(n), to distinguish the Virago and the RH/RJ from the rest of the world's TR1. When used in reference to intake, it indicates Naturally Aspirated in contra-distinction from blown, When NA is used, it should be instantly clear from context which meaning is to be understood.
USD: United States Dollars. If a number is given as a price, this is the unit that is meant -- whether explicitly stated or not. This is the case here and on the eCommerce sites, although the dollar sign ($) is used there as well. Similarly, NZD, AU$, and CAN$ should now be self-evident.
U-D: Upside Down. Referring to late-style forks where the "slider" is up top in the clamps, and the tubes are bolted to the axle.