The term Inner Circle is often used to refer to a select group of people who have a special position within an organization where they control, command or have access to, more resources, knowledge or power then the average person.
In Thief, there are two known groups using this term:
Hand Brotherhood[]
The Hand Brotherhood is governed by a council of arch mages who work to maintain order both within the Mage Towers[1] and abroad.[2] This group are masters in their field and all other mages must answer to them.
The City Nobility[]
Certain members of the upper class, such as Frederick Rothchild and his wife are referred to as belonging to a group known as the inner circle. The Bumblesons were once associated with them, but have not been active members in recent years.[3] As the Bumblesons appear somewhat petty, they may be just using the term as a way to look down on other minor nobles of the City.