The document discusses three key concepts of environmental impact assessment:
1) Defining "impacts" as changes to the baseline environmental situation caused by a proposed activity. The baseline is the existing condition without the activity.
2) Defining an "activity" as the specific project or program and associated sub-activities being assessed.
3) Characterizing the baseline situation, which involves understanding both current conditions and historical trends over time. Together these concepts form the foundation of assessing potential environmental effects.