A gravity wave was a wave of gravitic energy in hyperspace.
Numerous such waves could be found all over the non-Euclidean dimension, forming a great danger for spacecraft travelling under impeller drive. However, in 1273 PD, Dr. Adrienne Warshawski invented the Warshawski sail, a new configuration for the impeller nodes that created two sail-shaped force fields that allowed the ships projecting them to "ride" the gravity waves, making hyperspace travel less dangerous and also much faster. (UHH)