The manual for the LaTeX package TikZ v. 3.0.1a states (section 19.7 "Graph Operators, Color Classes, and Group Expressions", p. 280):
TikZ's graph command employs a powerful mechanism for adding edges between nodes and sets of nodes. To a graph theorist, this mechanism may be known as a graph expression: A graph is specified by starting with small graphs and then applying operators to them that form larger graphs and that connect and recolor colored subsets of the graph's node in different ways.
I searched Google for "graph expression" and couldn't find anything relevant. Where would a graph theorist know the term graph expression from? I'd appreciate references to where this term is defined, explained and used.