Index options on the S&P 100 are traded with the ticker symbol "OEX". Because of the popularity of these options, investors often refer to the index by its ticker symbol.
The S&P 100, a subset of the S&P 500, includes 100 leading U.S. stocks with exchange-listed options. Constituents of the S&P 100 are selected for sector balance and represent about 57% of the market capitalization of the S&P 500 and almost 45% of the market capitalization of the U.S. equity markets. The stocks in the S&P 100 tend to be the largest and most established companies in the S&P 500.
In 1983, the CBOE created the first index options, based on its own index, the CBOE 100.
In 1993, CBOE created the Chicago Board Options Exchange Market Volatility Index (VIX), which was computed based on the price of S&P 100 options (at the time these were by far the most heavily traded index options). Then in 2003, they changed it to be based on the S&P 500.
(as of March 21, 2014)
The average market capitalization (weighted by market capitalization) of the S&P 100 is about twice that of the S&P 500 ($142 bn vs $68 bn as of April 2014). So it is larger than a large-cap index. The "sigma" is usually a bit less, so it goes down less and goes up less. However, the correlation between the two is very high.