  Much of the baryonic matter in the Universe, including the most active and
luminous sources, are best studied in the X-ray band. Key advances in X-ray
optics and detectors have paved the way for the Advanced X-ray Imaging
Satellite (AXIS), a Probe-class mission that is a major improvement over
Chandra, which has generated a steady stream of important discoveries for the
past 2 decades. AXIS can be launched in the late 2020s and will transform our
understanding in several major areas of astrophysics, including the growth and
fueling of supermassive black holes, galaxy formation and evolution, the
microphysics of cosmic plasmas, the time-variable universe, and a wide variety
of cutting-edge studies. Relative to Chandra, the AXIS PSF is nearly twice as
sharp on-axis; its field of view for subarcsecond imaging 70 times larger by
area; its effective area at 1 keV is 10 times larger. The low-Earth orbit
ensures a low and stable detector background, resulting in 50 times greater
sensitivity than Chandra for extended sources. AXIS has a rapid repointing
response with operations similar to Swift, but is 100 times more sensitive for
time-domain science. These capabilities open up a vast discovery space and
complement the next generation of astronomical observatories. A
high-spectral-resolution mission (Athena) operating at the same time as a
high-angular-resolution mission (AXIS) greatly increases the range of
scientific discovery. AXIS will use lightweight X-ray optics made of thin
single-crystal silicon mirrors developed at NASA Goddard. The detector array
builds on a long legacy of X-ray CCD and provides improved photon localization,
much faster readout time, and broader energy band. The estimated mission costs
are consistent with the $1B Probe mission cost guideline.
