Supply and Demand: The fundamental forces that determine prices in a market economy; supply represents producers' willingness to sell at various prices while demand represents consumers' willingness to buy at various prices.
Opportunity Cost: The value of what must be given up when making choices due to scarcity; it represents the next best alternative foregone when making decisions.
Keynesian Economics: An economic theory developed by John Maynard Keynes during the Great Depression that advocates for government intervention through fiscal policies to stabilize the economy, particularly during times of recession.