  The physical origin of hysteresis in condensed matter had not been previously
identified. The current "science of hysteresis" is useful, but limited by
phenomenological modeling. This article fills the void by revealing the
exclusive cause of the hysteresis in structural, ferromagnetic and
ferroelectric phase transitions, as well as upon magnetization in magnetic
fields and polarization in electric fields. This exclusive cause is nucleation
lags. The lags are inevitable due to the nucleation specifics, far from the
classical "random fluctuation" model. A major assumption that spin orientation
is determined by the orientation of its carrier explains why ferromagnetic
transitions and magnetization in magnetic fields materialize by structural
rearrangements at interfaces, as well as why magnetization by "rotation" is
impossible. Formation of the structural and ferromagnetic hysteresis loops is
considered in detail.
