A sovereign default is the failure or refusal of the government of a sovereign state to pay back its debt in full when due. Cessation of due payments (or receivables) may either be accompanied by that government's formal declaration that it will not pay (or only partially pay) its debts (repudiation), or it may be unannounced. A credit rating agency will take into account in its gradings capital, interest, extraneous and procedural defaults, and failures to abide by the terms of bonds or other debt instruments.
Countries have at times escaped some of the real burden of their debt through inflation. This is not "default" in the usual sense because the debt is honored, albeit with currency of lesser real value. Sometimes governments devalue their currency. This can be done by printing more money to apply toward their own debts, or by ending or altering the convertibility of their currencies into precious metals or foreign currency at fixed rates. Harder to quantify than an interest or capital default, this often is defined as an extraneous or procedural default (breach) of terms of the contracts or other instruments.