Sentence:  Problems involving <WSD>graphs</WSD> first appeared in the mathematical folklore as puzzles (e.g. Königsberg bridge problem). Later, <WSD>graphs</WSD> appeared in electrical engineering (Kirchhof's Law), chemistry, psychology and economics before becoming a unified field of study. 
Meaning of the word marked with <WSD> tags:  An ordered pair of sets (V,E), where the elements of V are called "vertices" or "nodes" and E is a set of pairs (called "edges") of elements of V;  a set of "vertices" (or "nodes") together with a set of "edges" that connect (some of) the vertices.