Traditionally, the term broker in the context of securities has implied someone who acts in an agency or intermediary capacity. However, many brokers now act as principal or perform a dual capacity function, i.e. entering into agency or principal's transactions according to the needs or wishes of the particular client or situation.
Some brokers are also market makers or interbroker dealers.
Whatever capacity applies in any given situation should be clear from the agreed terms of trade between the broker and the particular client concerned.