Source : Webster's Revised Unabridged Dictionary (1913)

Onrush \On"rush`\, n.
   A rushing onward.

Source : WordNet®

onrush
     n 1: (military) an offensive against an enemy (using weapons);
          "the attack began at dawn" [syn: {attack}, {onslaught},
          {onset}]
     2: a forceful forward rush or flow; "from the bow she stared at
        the mesmerising onrush of the sea where it split and
        foamed"; "the explosion interrupted the wild onsrush of
        her thoughts"