(electromagnetic radiation in the range of 0.15-124 nm)
The term XEUV is used in astrophysics for an EMR spectrum-range
spanning extreme ultraviolet (EUV) plus some or all the X-ray range, definitely
including its less energetic portion (soft X-rays).
Papers using the term do not always specify the exact range
they mean, but when specified, the lower-wavelength bound
is generally in the 0.1 and 0.2 nm range. However, the term may
sometimes be intended to cover much or all of X-rays and/or
ultraviolet.
A term for such a range is useful because any defined boundary
between UV and X-rays is arbitrary;
given the (somewhat-consensus) boundary of 10 nm (or any other
defined boundary), there would be sources emitting EMR spanning
the boundary, some effects such as photoevaporation would
result from EMR on both sides, and some optics techniques
(e.g., sensing instruments) would be applicable to both.