adjacent

Adjacent is an adjective meaning “next to or nearest.” It describes things that are side by side or in close proximity. In everyday language you might refer to adjacent rooms, adjacent pages in a book, or an adjacent lot. The term can also be used as a noun in some contexts, though the adjective usage is far more common.

In geometry and mathematics, adjacent describes items that share a common vertex and a common side, lie in the same plane, and do not overlap. For example, adjacent angles share a vertex and a side; they do not overlap. If their non-common sides form a straight line, the measures of adjacent angles are supplementary and sum to 180 degrees.

In graph theory and computer science, adjacent vertices or nodes are connected by an edge. The concept is extended in data structures through adjacency lists and adjacency matrices, which record which pairs of elements are connected.

In geography and cartography, adjacent regions or countries share a boundary. Adjacency is symmetric: if region A is adjacent to region B, then region B is adjacent to region A. Adjacency relations underpin spatial analysis, mapping, and regional planning.

Etymology traces adjacent to the Latin adiacere, from ad- “near” plus iacere “to lie.” The related noun is adjacency, with adjacentness as an alternative form. The adverb adjacently also exists in standard usage.