Title Density is a Helium 10-specific metric surfaced in Blackbox's Keywords tab: it counts how many page-one listings for a given keyword contain that exact phrase, in that exact word order, in their listing title.
It's a distinct competitive signal from review count — a keyword can have real demand and light review competition per Helium 10 Blackbox Keywords Tab Filtering Protocol, yet still be hard to rank for if most page-one listings already carry the exact phrase in their titles. Setting a low max title density (e.g. 5) filters for keywords with little direct title competition, improving the odds of ranking page one with a well-optimized listing.
Relevant to on-page title construction once a keyword is chosen: Amazon Listing Title Structure Formula, A9 Title Keyword Weighting (Position & Phrase Match).