Amazon PPC glossary
Search term saturation
Whether a product's keyword list itself still has relevant, unclaimed search terms worth targeting, as distinct from impression share, which measures performance only on the terms already being targeted.
Where it matters most: Book 5, The Advanced PPC Clarity Guide, in the chapter “Scaling or Protecting a Winning Campaign.”
Go deeper
The glossary gives you the one-line answer. The Advanced PPC Clarity Guide explains why it matters and what to do about it.