To fail to meet an obligation.

“If you do not make your payments, you will default on your loan.”

To lose a competition by failing to compete.

“If you refuse to wear a proper uniform, you will not be allowed to compete and will default this match.”

To assume a value when none was given; to presume a tentative value or standard.

“If you don't specify a number of items, it defaults to 1.”

The act or result of being defaulted, especially a failure to meet a financial obligation.