Hello David
In the context of CEF, the term “adjacency” refers to a directly connected next-hop address or interface that a router uses to forward packets. Essentially, an adjacency is a neighbor device or network node that is one hop away and can be reached directly over a Layer 2 link.
The CEF adjacency table contains adjacencies, that is, entries of next-hop IP addresses (of adjacent routers). For each of those entries in the adjacency table, it includes the next hop IP address, the MAC address of the next-hop device, the egress interface, and any Layer 2 encapsulation information. That’s enough info to immediately forward any packet that matches an entry in the adjacency table without further processing.
For more info, take a look at this NetworkLessons note on the topic of the CEF Adjacency Table.
I hope this has been helpful!
Laz