Broker-dealer

When executing trade orders on behalf of a customer, the institution is said to be acting as a broker. When executing trades for its own account, the institution is said to be acting as a "dealer." Securities bought from clients or other firms in the capacity of dealer may be sold to clients or other firms acting again in the capacity of dealer, or they may become a part of the firm's holdings.
A broker-dealer is a company or other organization that trades securities for its own account or on behalf of its customers.
Although lots of broker-dealers are "independent" firms solely involved in broker-dealer services, lots of others are business units or subsidiaries of commercial banks, investment banks or investment companies.

Regulation

United States
The 1934 Act defines "broker" as "any person engaged in the business of effecting transactions in securities for the account of others," & defines "dealer" as "any person engaged in the business of buying & selling securities for his own account, through a broker or otherwise." Under either definition, the person must be performing these functions as a business; if conducting similar transactions on a private basis, they're considered a trader & subject to different requirements
In the United States, broker-dealers are regulated under the Securities Exchange Act of 1934 by the Securities & Exchange Commission (SEC), a unit of the US government. Some regulatory authority is further delegated to the Financial Industry Regulatory Authority (FINRA), a self-regulatory organization. lots of states also regulate broker-dealers under separate state securities laws (called "Blue sky laws").

United Kingdom
The Financial Services Authority authorises & regulates companies engaging in such activity as "regulated activities" under the Financial Services & Markets Act 2000.
UK securities law uses the term intermediary to refer to businesses involved in the purchase & sale of securities for the account of others.

Japan
The common Japanese term for a broker-dealer is "securities company" (証券会社, shōken-gaisha?). Securities companies are regulated by the Financial Services Agency under the Financial Instruments & Exchange Law. The "big three" are Nomura Holdings, Daiwa Securities Group & Nikko Cordial (a subsidiary of Citigroup). Most major commercial banks in Japan also maintain broker-dealer subsidiaries, as do plenty of foreign commercial banks & investment banks.
Securities companies must be organized as kabushiki kaisha with a statutory auditor or auditing committee, & must maintain minimum shareholder equity of ¥50 million.