1. In the most general context, a brokerage firm (or broker) holding membership on an organized stock or commodities exchange. Membership is generally required in order to fill trades for clients on the exchange. 2. For the New York Stock Exchange, one of more than 1,300 individuals or firms owning a seat on the exchange. 3. For the National Association of Securities Dealers (NASD), any broker-dealer admitted to membership in the association. Taobiz explains Member Brokerages must have a "member seat" on the NYSE to trade stocks for their clients. The larger firms have several seats on the exchange. Seats cost more than $1 million each!