noun
default; nonperformance; nonfulfillment
Legal and formal term for failure to perform an obligation, such as a contract or duty.
契約の不履行は損害賠償の対象となる。
Nonperformance of a contract may result in liability for damages.
債務不履行で
He was sued for default on his debt.
Means 'performance' or 'fulfillment'; 不履行 is its direct antonym.
Compound of 不 (non-) and 履行 (performance, fulfillment). The latter is from Chinese origin, used in legal and formal Japanese.