What is a "wave election?"

As in "Democrats spread senate map in places unlikely to flip their way even if it becomes a wave election in their favor. Beyond their top offensive targets in states like Maine, North Carolina, Arizona and Colorado, or expansion targets like Iowa, Montana and Georgia, Democrats want to drain Republican coffers in states where the GOP doesn’t want to have to spend money: South Carolina, Kentucky, even Mississippi"

From the WSJ "the wire"

https://www.wsj.com/articles/democratic-dollars-force-republican-spending-in-red-states-11595583000 (subscription)