A device configured for IS-IS forms an adjacency with each of the IS-IS devices to which it is directly connected. An adjacency is a two-way direct link (a link without router hops) over which the two devices can exchange IS-IS routes and other protocol-related information. The link is sometimes called a “circuit”. The devices with which the device forms adjacencies are its neighbors , which are other ISs.
In Intermediate systems and end systems, Router A has an IS-IS adjacency with Router B. Likewise, Router B has an IS-IS adjacency with Router A and Router C.