A block trade alert means that there was a single trade with at least 20,000 shares.  Block trades usually show institutional trading.  These large trades are done over the phone.  The traders report them electronically after the fact.  If a trader tries to make a large trade on an ECN, the trade will usually be broken into many smaller prints, rather than one large one.

The description of the alert may contain additional information:

• At the ask - The price of the trade was the current ask price.
• At the bid - The price was the current bid price.
• Trading above - The price was higher than the current ask price.
• Trading below - The price was lower than the current bid price.
• Trading between - The price was between the bid and the ask.

The description also includes the name of the exchange where the trade takes place, when that information is available.

Note:  This was previously known as a "block print" alert.

More options related to this alert are listed below.