The Collaborative International Dictionary of English v.0.48:

Adjacence \Ad"ja"cence\, Adjacency \Ad*ja"cen*cy\, [Cf. LL.
   adjacentia.]
   1. The state or attribute of being adjacent or contiguous;
      contiguity; the attribute of being so near as to be
      touching; as, the adjacency of lands or buildings.

   Syn: contiguousness
        [1913 Webster]

   2. That which is adjacent. [R.] --Sir T. Browne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

adjacency
    n 1: the attribute of being so near as to be touching [syn:
         adjacency, contiguity, contiguousness]

The Free On-line Dictionary of Computing (30 December 2018):

adjacency
adjacent

    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).

   (1998-03-10)