One indication of a blow-off top is when exchange-traded fund buying gets crazy. This was the case ahead of the last week's selloff.
The overbought market must pull back eventually, but it won't necessarily be tomorrow, next week, or even next month.
The S&P 500 is a market-cap-weighted index of 505 large-cap U.S. stocks, representing approximately 80% of the market value of the U.S. stock market. Often synonymous with "the market" in the U.S., the S&P 500 is the closest thing to a default U.