From Traderpedia

Definition: A method of public auction, by which competing market makers, brokers and floor traders (locals) shout and signal their bids, offers and trades on the trading floor of the exchange.

Currently still in use at most US commodity and futures exchanges, where trading occurs on a trading floor and traders may bid and offer simultaneously either for their own accounts or for the accounts of customers. Transactions may take place simultaneously at different places in the trading pit or ring.

At the majority of exchanges outside the US, open outcry has been replaced by fully electronic trading platforms where orders are sent by computer and matched by an electronic matching engine at the exchange. Electronic trading allows for greater execution speeds and volume of trade, along with greater transparency. Most recently London's International Petroleum Exchange closed its trading floor in April 2005 in favour of fully electronic order matching, the only remaining open outcry in Britain is conducted at the London Metals Exchange.