What is the meaning of trench warfare?

noun

• A struggle (usually prolonged) between competing entities in which neither side is able to win.
  • usage: "the hope that his superior campaigning skills would make a difference evaporated in the realization that electioneering had become a form of trench warfare"
• A type of armed combat in which the opposing troops fight from trenches that face each other.
  • usage: "instead of the war ending quickly, it became bogged down in trench warfare"