Definition from Nolo’s Plain-English Law Dictionary
A statement, usually written, from one party to a contract to another, advising the recipient that he or she has failed to live up to a term or condition of the contract. Although defaults are most common in real estate, they can happen in any contract situation. A notice of default can result when a homeowner fails to pay as required under his mortgage or deed of trust. In a lease situation, a notice of default can be sent by either the landlord or tenant, claiming that one of them has violated a condition of the lease. If the defaulting party fails to correct, or cure, the default, the other side may declare the contract to be over, by instituting foreclosure proceedings, an eviction, or taking any other act, depending on the nature of the contract, that ends the contractual relationship.