Simply put, beta (ß) is a measure of market risk. More precisely, it is a measure of the volatility—or systematic risk—of a security or portfolio compared to the market as a whole. In statistical terms, it is the slope of the coefficient for a security (stock) regressed against a benchmark market index (S&P 500). Each of these data points represents an individual stock's returns against those of the market as a whole. So, beta effectively describes the activity of a security's returns as it responds to swings in the market.