  We discuss the dispersion relations of Nambu-Goldstone (NG) modes associated
with spontaneous breaking of internal symmetries at finite temperature and/or
density. We show that the dispersion relations of type-A (I) and type-B (II) NG
modes are linear and quadratic in momentum, whose imaginary parts are quadratic
and quartic, respectively. In both cases, the real parts of the dispersion
relations are larger than the imaginary parts when the momentum is small, so
that the NG modes can propagate far away. We derive the gap formula for NG
modes in the presence of a small explicit breaking term. We also discuss the
gapped partners of type-B NG modes, when type-A and type-B NG modes coexist.
