Amazon PPC glossary
Ceiling
A limit on a product's growth set by the market, the category, or the listing itself, rather than by anything fixable in the campaign.
Where it matters most: Book 5, The Advanced PPC Clarity Guide, in the chapter “When a Product Reaches Its Ceiling.”
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.