  This article describes BabyIAXO, an intermediate experimental stage of the
International Axion Observatory (IAXO), proposed to be sited at DESY. IAXO is a
large-scale axion helioscope that will look for axions and axion-like particles
(ALPs), produced in the Sun, with unprecedented sensitivity. BabyIAXO is
conceived to test all IAXO subsystems (magnet, optics and detectors) at a
relevant scale for the final system and thus serve as prototype for IAXO, but
at the same time as a fully-fledged helioscope with relevant physics reach
itself, and with potential for discovery. The BabyIAXO magnet will feature two
10 m long, 70 cm diameter bores, and will host two detection lines (optics and
detector) of dimensions similar to the final ones foreseen for IAXO. BabyIAXO
will detect or reject solar axions or ALPs with axion-photon couplings down to
$g_{a\gamma} \sim 1.5 \times 10^{-11}$ GeV$^{-1}$, and masses up to $m_a\sim
0.25$ eV. BabyIAXO will offer additional opportunities for axion research in
view of IAXO, like the development of precision x-ray detectors to identify
particular spectral features in the solar axion spectrum, and the
implementation of radiofrequency-cavity-based axion dark matter setups.
