Regulators are probing block trading on Wall Street, but what is a block?

The U.S. Securities and Exchange Commission is investigating whether certain banks may have broken the rules by tipping off hedge funds ahead of large sales of shares, or "block trades," a source familiar with the situation told Reuters on Tuesday.

The scrutiny is part of a years-long investigation into block trading by some of Wall Street's biggest players. Here is a primer on block trading.

What is a Block Trade?

The New York Stock Exchange defines a block as at least 10,000 shares or a quantity of stock having a market value of $200,000 or more, whichever is less.

If a seller such as