Bob Keefer , May 22, 2010; 12:36 p.m.
There is no clear definition.
Generally, though, camera manufacturers produce a line of zooms aimed at newspaper, sports and wedding photographers; the lenses are fast -- f/2.8 -- and well built. The Pentax DA* 16-50/2.8 and its DA* cousins fall in this category.
Companies also often produce premium primes, with solid construction and better optics than the usual run.
Canon marks its top lenses as "L" and gives them a red ring.
I'm not familiar enough with Nikon to attempt to describe their system.
Pentax has long used a star, or "*," for its premium lenses. Pentax also has a separate line of "Limited" lenses that is unlike anything from Canon or Nikon. The Limiteds are small, good quality primes that are not always very fast (that's the tradeoff for small) but make a good companion to Pentax's small bodies.
Hope this helps.