Value at Risk
Value at risk, or VaR, is a widely used measure of tail risk. If the 1-day 95% value at risk for a portfolio is $4 million, then there is a 95% chance that the portfolio will not lose more than $4 million, and a 5% chance that it will lose $4 million or more.
There are many different ways to calculate VaR including the delta-normal approach, historical simulation, the hybrid approach, and Monte Carlo simulation.
For more information on VaR models, see our white paper, An Introduction to Value at Risk.