This term classically refers to bilateral trading between counterparts. Counterparts extend each other credit, making an assessment of the counterpart’s default risk internally. This type of trading can be facilitated in theory by any trader; however, it is commonly an investment bank that provides this facility to give traders access to the market. Confusingly, you will also hear the term ‘OTC market’ to define the oil swaps market, as the trading is still mostly voice-executed via brokers who then pair two traders together. Historically the swap market was only accessible by trading bilaterally, and the terminology has stuck.