U.S. stocks were slammed on Thursday, with high-flying technology and biotech shares leading the declines that had the Nasdaq Composite posting its worst session in more than two years.

"The market is coming to its senses in some of the high-flying tech names; it looked like there were some pretty hefty amounts being paid for the prospect of eventual earnings. Any of us in the market more than 15 years feels the hot breath on the backs of our necks when we see such high prices being paid for tech stocks," said Jerry Webman, chief economist at Oppenheimer Funds.