An adequacy decision confirms that a third country has a level of data protection comparable to that of the EU.
An adequacy decision is a ruling by the European Commission according to Article 45 GDPR that confirms a third country, international organization, or specific sector offers an adequate level of data protection, i.e. a level that is comparable to that in the EU. This means that personal data can be transferred from EU countries to these entities without additional safeguards being required. One can think of that as extending the common data space beyond the EU and the European Economic Area (EEA).
For countries or organizations that are not covered by an adequacy decision, other transfer tools have to be used to transfer data, such as Standard Contractual Clauses (SCC).