Buzzword: Dark Pools

What it means: Trades between large-scale institutional investors that don't show up on public exchanges. Many observers of last week's stock-market tumble—nearly 1,000 points off the Dow Jones Industrial Average in just 20 minutes—now believe that such dark-pool activities, which couldn't be viewed by all market participants, contributed to the volatility.

Another possible source of the trouble: When the markets ran amok on Thursday, the New York Stock Exchange initiated pre-set circuit breakers that slowed computer-programmed trades, but smaller, electronic-based exchanges did not. The trades that continued to take place had fewer matches between buyers and sellers, resulting in deep plunges in individual stocks.

The House Financial Services Capital Markets Subcommittee will hold hearings tomorrow to probe what happened. Officials from the Securities and Exchange Commission, Commodities Futures Trading Commission, and representatives of major exchanges will be among those expected to testify.

Given the timing of the market's scary plunge and quick rebound, it's no surprise that lawmakers wrestling with systemic financial reform legislation have weighed in on new measures to reduce the likelihood of another such instant plunge. Read Bloomberg.com's account of some senators' responses.