The marketplace where governments and corporations borrow by selling tradeable promises to repay with interest, and where the price of that borrowing is continuously set. The bond market turns a loan into a security that can change hands, prices a government's credibility every day, and exercises a discipline over states that the stock market does not — which is why it has often mattered more than the exchange where shares trade.
In 1993, a political adviser to a newly elected American president joked that he wanted to be reborn as the bond market, because then he could intimidate anybody. He was not really joking. The bond market is where governments go to borrow, and the price it charges them is a daily verdict on whether their promises can be trusted. It cannot vote a leader out. It does something quieter and harder to ignore. It raises the cost of every road, war, and pension a government wants to finance, until the policy itself becomes too expensive to keep.