An X-ray flash (XRF, aka X-ray outburst, XRO) is a short
flash of X-rays, much like a gamma-ray burst (GRB).
In fact, it is suggested that (at least some) X-ray flashes and
gamma-ray bursts stem from the same type of mechanism, i.e.,
a mechanism producing frequencies in a range spanning the boundary
between X-rays and gamma rays.
One theory is that the difference between gamma-ray bursts and X-ray
flashes is our viewing angle, i.e., that an X-ray flash seen from
some directions (that are far away from our view from the solar system)
would appear as a GRB, and vice versa.