No Dealing Desk - a broad category covering STP and ECN brokers that do not pass orders through an internal dealing desk.
NDD brokers pass orders straight to liquidity providers (STP) or to an electronic matching pool (ECN). The absence of a dealing desk removes the conflict of interest inherent in a market-maker model.
NDD is a marketing label rather than a regulated category, so traders should verify execution model details in the broker's order execution policy document.