What is a bond default?
Bonds are a type of loan that an investor makes to a company. In return, the company pays back the principal, as well as interest. The interest can be paid when the bond matures, while the bond is held, or both.
When a bond defaults, it's due to at least one payment not being met. So, although a bond default sounds pretty terrifying, it could simply be a failure of a company to make a single interest payment, which it later catches up, or it could be as severe as the company failing to pay for bonds at maturity.