Specialist

What Does Specialist Mean?
A member of an exchange who acts as the market maker to facilitate the trading of a given stock. The specialist holds an inventory of the stock, posts the bid and ask prices, manages limit orders and executes trades. Specialists are also responsible for managing large movements by trading out of their own inventory. If there is a large shift in demand on the buy or sell side, the specialist will step in and sell out of their inventory to meet the demand until the gap has been narrowed.

Investopedia explains Specialist
There is usually one specialist per stock who stands ready to step in and buy or sell as many shares as needed to ensure a fair and orderly market in that security.