Sentence:  <WSD>Euler lines</WSD> mainly deal with the nature of connectivity in graphs. The concept of an <WSD>Euler line</WSD> is used to solve several puzzles and games....A closed walk running through every edge of the graph "G" exactly once is called an <WSD>Euler line</WSD>. 
Meaning of the word marked with <WSD> tags:  (graph theory) An Eulerian path, a looped path through a graph that passes along every edge exactly once.