A citation gap is the space between the sources AI engines actually cite when answering your category's questions and the sources that include you. Run the audit and it shows where the answer is being built from pages you're not on. The specific places winning the citations you're missing.
It comes in two related lenses. A source gap asks: which domains and pages do the engines repeatedly cite for my topic, and am I present on (or referenced by) them? A mention gap asks: in answers where competitors appear, why am I absent? Both turn a vague "we're not very visible" into a concrete target list.
It's one of the most actionable things to measure because it points straight at work: the third-party lists to earn placement in, the questions your own pages don't yet answer, the corroborating coverage you lack. Tracking it alongside citation coverage and share of voice turns AI-visibility monitoring into a prioritized to-do list rather than a scoreboard.