Items that cannot be sold to a vendor NPC for money, disenchanted, or used for a quest, are definitely junk.

One definition includes more:
  • Items you do not plan to use, but just sell to a vendor for money or just trash (throw away). Also known as vendor trash.
Alternative definition:
  • Items that cannot even be sold to a vendor. This perhaps does not apply so much to WoW, since there are very few items that cannot be sold for at least pocket change. In a broader use among MMORPGs in general though, this is perhaps more common.
  • Junk can also be used as a verb meaning the process of selling stuff for money OR simply throwing them away (more commonly the latter in WoW in the author's experience), as in: "I'm just going to junk this staff, since I can't use it."
  • "Junk" is also a profession-category in Engineering. The only recipe listed in it is  [Practice Lock].