The time or period of time on which an index is based. Usually, the index is established at a value of 10 or 100 in the base year. The consumer price index of 220 in mid-2008 indicated that consumer prices were 120% higher than during the base period of 1982-84. The Standard & Poor's 500 Stock Index of 1275 in mid-2008 indicated that, on average, a portfolio of blue chip stocks with a market value of $10 during the period 1941-43 (the base period) had a market value of $1,275 in mid-2008. The portfolio of stocks making up the S&P 500 underwent substantial changes during the intervening years.