Delivery versus payment (DvP)
The rule that the security and the money change hands at the same moment or not at all, so nobody hands over their half and walks away empty-handed.
The risk it removes is principal risk: the buyer pays and the seller never delivers, or the other way round. In 1992 the BIS committee on payment systems described three models that are still the standard reference. Model 1 settles both securities and cash gross, trade by trade. Model 2 moves securities gross but nets the cash at the end of the day. Model 3 nets both. More netting means less liquidity tied up, and a bigger mess if a participant can't cover its net position.
In Europe, DvP in central bank money has largely run on TARGET2-Securities since 2015, with securities accounts and cash on the same platform. The currency-market equivalent, where both legs are money, is payment versus payment, which is the whole reason CLS exists.