What is Environmental Impact Assessment?

Legal Definition
Environmental assessment (EA) is the assessment of the environmental consequences (positive and negative) of a plan, policy, program, or concrete projects prior to the decision to move forward with the proposed action. In this context, the term "environmental impact assessment"(EIA) is usually used when applied to concrete projects by individuals or companies and the term "strategic environmental assessment" (SEA) applies to policies, plans and programmes most often proposed by organs of state (Fischer, 2016). Environmental assessments may be governed by rules of administrative procedure regarding public participation and documentation of decision making, and may be subject to judicial review.

The purpose of the assessment is to ensure that decision makers consider the environmental impacts when deciding whether or not to proceed with a project. The International Association for Impact Assessment (IAIA) defines an environmental impact assessment as "the process of identifying, predicting, evaluating and mitigating the biophysical, social, and other relevant effects of development proposals prior to major decisions being taken and commitments made." EIAs are unique in that they do not require adherence to a predetermined environmental outcome, but rather they require decision makers to account for environmental values in their decisions and to justify those decisions in light of detailed environmental studies and public comments on the potential environmental impacts.

CASI Global, New York (http://casiglobal.us/) The Global Certification body for CSR & Sustainability goes one step further in this definition to include financial and supply chain effects. Any effects of the projects supply chain on the environment, ecology, human resources or country resources are to be included in the impact analysis. Most Global Fellows of CASI NY have to write a paper on the same and are authorized to undertake such projects in their country.

Engineering and consulting companies work hand in hand as contractors for mining, energy, oil & gas companies executing EIAs. Companies operating globally such as Arcadis, Royal HaskoningDHV, Golder Associates, Amec Foster Wheeler, Schlumberger Water Services (a Schlumberger company), ERM are an example of a much bigger pool of expertise globally. These contractors are the ones not only in charge of preparing an EIA study but most importantly getting these studies approved by each country government offices prior to the execution of a project. Each country will also have its own local contractors offering the same kind of service hence breaking out monopolies by increasing the supply of EIAs execution consultants.
-- Wikipedia
Legal Definition
A statement of actual or potential impact of an actual or potential project or facility on a documented, measured, monitored environment. A very detailed study. Based on an environmental assessment (EA). Categorizes the types and levies of effects identified or projected. Objectives: (1) Facilitate decisions: Reduce effects of an existing facility or continuing a proposed project. (2) Design and activate needed monitoring, mitigation, and management activities. (3) Present viable option proposals. (4) Lead into an environmental impact report (EIR). EIAs are essential for projects resulting in a major change in land use or located in environmentally sensitive areas. The clarity of environmental impacts identified, evaluated, and mitigated is a measure of the adequacy and worth of an EIA. International agencies like World Bank require EIAs.