Broker
Strictly speaking, an investment firm which handles client orders for transactions in securities as an agent. To be contrasted with a dealer, who trades in the market as a principal in its own right.
More commonly referred to as a broker-dealer, seeing as most firms act as both.
Curiously one may speak of an executing broker but not an executing dealer, and even if you did, you would be talking of a quite different beast to a prime broker, which is neither, really, a broker or a dealer.