Definition of 'Defunct'
The condition of a company, whether publicly traded or private, that has gone bankrupt and ceased to exist. If the company was publicly traded, it will be delisted from the exchange where it was listed, and its stock will be worth nothing.
This term also applies to currencies that are no longer in circulation.
Investopedia explains 'Defunct'
In exceptional situations, the Securities and Exchange Commission does allow the stock of a defunct company to continue to trade for a period of time, but this is as a result of legal technicalities rather than a particular right or privilege.
History has seen many defunct currencies (e.g. the Greek drachma and the Dutch guilder). Currencies can become defunct for many reasons - for example, due to political upheaval or revolution, or because the currency has become worthless in the foreign exchange market.