A sailing technique where the mainsail is extended out one side perpendicular to the center line of the boat and the jib is extended out the other side. The overall appearance is like a pair of outspread wings. This is useful when running before the wind. It's not as efficient as using a spinnaker but much easier than handling one.
To effectively run wing on wing, a whisker pole is necessary. This holds the jib out into clean air, making it more stable and efficient.