What does Boxing Day have to do with boxing?
A relationship between two network devices, e.g. routers, which are connected by one media segment so that a packet sent by one can reach the other without going through another network device. The concept of adjacency is important in the exchange of routing information.
Adjacent SNA nodes are nodes connected to a given node with no intervening nodes. In DECnet and OSI, adjacent nodes share a common segment (Ethernet, FDDI, Token Ring).