The Collaborative International Dictionary of English v.0.48:

Onrush \On"rush`\, n.
   A rushing onward.
   [1913 Webster]

WordNet (r) 3.0 (2006):

onrush
    n 1: (military) an offensive against an enemy (using weapons);
         "the attack began at dawn" [syn: attack, onslaught,
         onset, onrush]
    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 onrush of her thoughts"

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "onrush":
   Brownian movement, advance, affluence, afflux, affluxion,
   angular motion, ascending, ascent, axial motion, backflowing,
   backing, backward motion, career, climbing, concourse, confluence,
   conflux, course, crosscurrent, current, defluxion, descending,
   descent, downflow, downpour, downward motion, drift, driftage,
   ebbing, flight, flow, flowing, fluency, flux, forward motion, gush,
   inflow, mill run, millrace, mounting, oblique motion, ongoing,
   onward course, outflow, passage, plunging, progress, race,
   radial motion, random motion, reflowing, refluence, reflux,
   regression, retrogression, rising, run, rush, set, sideward motion,
   sinking, soaring, spate, sternway, stream, subsiding, surge, tide,
   traject, trajet, trend, undercurrent, undertow, upward motion,
   water flow