1 failure, fault, defect, neglect, negligence, dereliction, lapse, oversight, non-performance, non-fulfilment, inaction:
We won the case because of their default on the contract. The other contestant failed to appear, so Gordon won the match by default.
2 non-payment, delinquency:
Default in the rent may result in eviction.
3 fail, neglect, dishonour, lapse, fall short, come (up) short:
He has defaulted on a car payment.