Beta is a "relative" measure of volatility when measuring a stock or fund against a set market index, in this case the S&P 500. Whether the S&P 500 goes up 1%, 5% or 50% the S&P 500 has a constant Beta value of "1". So, "Beta" is a value assigned to a company based on how it performs over time compared to the S&P 500.
So, for example, if the price of Company A's stock - over a period of time - tends to go up by 2% each time the S&P goes up 1%, the Beta of Company A will be "2". Conversely, if the S&P 500 falls by 1%, Company A will decline by 2%. The higher a company's Beta value is, the more volatile the price deviations will be when compared to the S&P 500 percentile moves.
A company with a high beta value above "1" might describe a young promising but unproven company with high growth prospects. As the overall market goes up, it's value goes up by a multiple of the percentage move in the S&P 500.
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