  We report an excellent realization of the highly non-classical incommensurate
spin-density wave (SDW) state in the quantum frustrated antiferromagnetic
insulator Cs$_2$CoBr$_4$. In contrast to the well-known Ising spin chain case,
here the SDW is stabilized by virtue of competing planar in-chain anisotropies
and frustrated interchain exchange. Adjacent to the SDW phase is a broad $m =
1/3$ magnetization plateau that can be seen as a commensurate locking of the
SDW state into the up-up-down (UUD) spin structure. This represents the first
example of long-sought SDW-UUD transition in triangular-type quantum magnets.
