Bot wall
A bot wall is any protection layer that stops an automated client before the storefront renders: an interactive challenge, an interstitial page, an outright block, or a rate limit that returns nothing useful.
From the store’s side this looks like protection working. From the agent’s side there is no store: it received a challenge page, and a challenge page has no products in it. The agent does not retry with a different opinion of you, because it has no opinion of you to revise.
The reason this is so often unintentional is that the rule blocking the agent is usually a default, set once when the protection was installed, and it makes no distinction between a scraper and a client shopping for a named customer.
Why it matters to a store
This is the failure that hides every other one. A scan that hits a bot wall cannot tell you anything about your product data or your checkout, because it never saw them.