I'd assume it's the same idea as the optical coherence length for photons, but applied to neutrinos.
To quote the most relevant part of the wikipedia entry (there is a lot of fluff)
The coherence length is the optical path length difference of a self interfering laserbeam which corresponds to a 50% fringe visibility, where the fringe visibility is defined as V = (Imax - Imin)/(Imax + Imin) and I is the fringe intensity.
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Note that optical coherence is used in stellar interferometery as well as lasers - the key idea is that photons will have visible interference fringes as long as the difference in path lengths is not too great. The coherence length is a measure of the maximum path length difference at which fringes will still occur.
I would assume that the concept applies in the same manner to neutrinos. Unfortunately, while neutrinos should theoretically interfere with themselves as do photons, it would be a lot harder to verify this via experiment as they interact so weakly.
