An environmental and social impact assessment (ESIA) is the study that identifies how a project will affect the environment and the people living in and around its area, and sets out how those impacts will be avoided, reduced or compensated. It is prepared before the project is approved.
An ESIA report usually covers the project and its alternatives, the environmental and social baseline, the predicted impacts, the consultation with affected people, and an environmental and social management and monitoring plan (ESMMP).
An environmental impact assessment (EIA) is the same kind of study. The name ESIA makes explicit that social impacts are assessed as well. In Lao PDR the assessment is required by the Decree on Environmental Impact Assessment, No. 21/GoL (2019), and the report is reviewed by the government's environment authorities before the project is approved. International lenders ask for one under IFC Performance Standard 1.
The ESIA and the resettlement action plan (RAP) are separate documents that work together. The ESIA identifies all the impacts of a project. Where it finds that land will be taken or people will have to move, the RAP sets out in detail who is affected, what they are entitled to, and how they will be compensated, relocated and helped to restore their livelihoods. The census and socio-economic survey are often used by both.