The Elementary and Secondary Education Act (ESEA), reauthorized in 2015 as the Every Students Succeeds Act (ESSA), emphasizes—and in some instances requires—the use of evidence-based activities, strategies, and interventions to improve student outcomes. Interventions that are evidence-based fall into one of four tiers. Each is defined by the type of evidence needed to qualify an intervention as "evidence-based" for that tier.
IES' Regional Educational Laboratories program has released a video and accompanying handout (PDF; 650kb) that further explain each of ESSA's four evidence tiers.
You can also learn more by reading the ESEA itself. According to Section 8101(21)(A) of the ESEA, an evidence-based intervention is one that:
Finally, the Department, as part of the Education General Administrative Regulations (EDGAR; see 34 CFR 77.1), further defines "well-designed and well-implemented" experimental, quasi-experimental, and correlational studies as part of its definitions of strong, moderate, and promising evidence.
Note: Some ESEA programs, including school improvement programs described in Section 6303 of the ESEA, allow evidence-based programs at the strong, moderate, or promising levels only. For more information, see Section 8101(21)(B) of the ESEA.