Amazon PPC glossary
Lead metric
The one metric currently deciding whether a product's PPC is working, chosen deliberately for the product's current stage rather than defaulted into.
Where it matters most: Book 5, The Advanced PPC Clarity Guide, in the chapter “Choosing Your Target Metric.”
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.