Specialist

A specialist was a designated person on the floor of a stock exchange, most famously the New York Stock Exchange, who was responsible for a specific group of stocks. Their job was to keep an orderly market in those names: matching buyers with sellers, and stepping in with their own firm's money to buy or sell when there was a temporary imbalance, such as far more sellers than buyers at the open.

To do this, the specialist kept the "book" — the running list of buy and sell orders sitting above and below the current price that hadn't been filled yet. When a stock opened for trading, or when there was a sudden imbalance during the day, the specialist would set a price that tried to clear as much of that book as possible, and would use their own inventory of shares to smooth over gaps in supply or demand.

The nuance that trips people up is that this was a physical, human-run role tied to floor-based trading, and it has largely been replaced. Exchanges have moved to electronic matching systems, and the specialist's functions — quoting prices, providing liquidity, managing the order book — are now largely performed by designated market makers (DMMs) and automated systems, with far less discretion and floor presence than the old specialist had. You'll still see the word used loosely, sometimes interchangeably with "market maker," but on modern exchanges the formal specialist role, as it once existed, is mostly historical.