  For S=1 system with general isotropic nearest-neighbor exchange, we derive
the low-energy description of the spin nematic phase in terms of the RP^{2}
nonlinear sigma-model. In one dimension, quantum fluctuations destroy
long-range nematic (quadrupolar) ordering, leading to the formation of a gapped
spin liquid state being an analog of the Haldane phase for a spin nematic.
Nematic analog of the Belavin-Polyakov instanton with \pi_{2} topological
charge 1/2 is constructed. In two dimensions the long-range order is destroyed
by thermal fluctuations and at finite temperature the system is in a
renormalized classical regime. Behavior in external magnetic field is
discussed.
