The amount of information presented per unit of screen space, set by type size, spacing, and how many elements share the view at once.
A trading dashboard earns its density. Traders want forty signals on screen, not four cards.
Density is a contract with the user, not a failure of design. Power tools should be dense: terminals, spreadsheets, consoles. Onboarding should not. The mistake is uniformity: consumer airiness applied to expert tools, or expert density on a first-run screen. Density and whitespace are the same dial read from opposite ends.