  Motivated by the geometry of the materials Na_2 Ti_2 As_2 O and Na_2 Ti_2
Sb_2 O, we study a square-lattice Heisenberg antiferromagnet with spins located
at the bond-centers. The largest exchange constant J couples neighboring spins
in a given row or column. This leads to a mesh of isolated spin-chains running
along the X and Y axes. A weaker exchange constant J' couples the
nearest-neighbor spins on the lattice. Classically, J' fails to fix the
relative spin orientation for different chains and hence the ground state is
highly degenerate. Quantum order by disorder effect is studied by spin-wave
theory and numerical methods. It is shown that a 4-sublattice order is favored
by quantum fluctuations. However, several arguments are presented that suggest
that the ground state of the system remains disordered, thus providing us with
a paradigm for a two-dimensional spin-liquid.
