A vast storm of radiolytic particles moving at warp speed, lethal to unshielded crews, which forced the company of Enterprise NX-01 to shelter for eight days inside a warp nacelle in The Catwalk.
A neutronic wavefront is a deep-space storm of intense radiation, light-years across and travelling faster than a ship of the 2150s can outrun. Ordinary hull plating gives no protection against it.
In the Enterprise episode The Catwalk in 2152, three aliens come aboard to warn of a class five wavefront bearing down on the ship. The only part of Enterprise shielded heavily enough is the maintenance shaft that runs the length of each nacelle, lined with osmium alloy. The entire crew, with the visitors, pack into one catwalk for the eight days the crossing will take, with the warp engine shut down because the shaft becomes unbearably hot when it runs.
The visitors turn out to be deserters from the Takret militia, whose former comrades board the apparently empty ship during the storm. Archer bluffs them into leaving by threatening to fly Enterprise into a plasma eddy.