An “adequacy decision” is a decision adopted by the European Commission on the basis of Article 45 of the GDPR, which establishes that a third country (i.e. a country not bound by the GDPR) or international organisation ensures an adequate level of protection of personal data. Such a decision takes into account the country's domestic law, its supervisory authorities, and international commitments it has entered into.
The effect of such a decision is that personal data can flow from the EU Member States and the European Economic Area member countries to that third country, without any further requirements. The European Commission publishes a list of its adequacy decisions on its website.