  A disordered material that cannot relax to equilibrium, such as an amorphous
or glassy solid, responds to deformation in a way that depends on its past. In
experiments we train a 2D athermal amorphous solid with oscillatory shear, and
show that a suitable readout protocol reveals the shearing amplitude. When
shearing alternates between two amplitudes, signatures of both values are
retained only if the smaller one is applied last. We show that these behaviors
arise because individual clusters of rearrangements are hysteretic and
dissipative, and because different clusters respond differently to shear. These
roles for hysteresis and disorder are reminiscent of the return-point memory
seen in ferromagnets and many other systems. Accordingly, we show how a simple
model of a ferromagnet can reproduce key results of our experiments and of
previous simulations. Unlike ferromagnets, amorphous solids' disorder is
unquenched; they require "training" to develop this behavior.
