Forex Prime Brokers are the biggest liquidity providers for currency transactions. There are only a few large banks that are real liquidity providers in the currency market, Bank of America, Barclays Capital, Morgan Stanley, Deutsche Bank and others. Most other brokers and traders forward orders into these systems or hedge against these quotes. The prime brokers only deal with other large financial institutions and not directly with any smaller customers and clients.
In a more general sense in the investment space, a Prime Broker, also Primer Brokerage, may be a broker to a hedge fund or other large trading institution. The Prime Broker is usually a large bank or an investment company providing a wide range of services to hedge funds related to clearing, operational support, settlement of transactions and risk management.