One way slab:

1. $Ly/lx\gt 2$

2. Slab is supported on two opposite parallel edges.

3. It spans only in one direction perpendicular to the supporting edges.

4. It bends in one direction.

5. Main steel is provided in one direction of span.

Two way slab:

1. $Ly/lx\lt= 2$

2. Slab is supported along all 4 edges

3. Supports are perpendicular to each other.

4. It bends in two direction at right angles to the supporting edges.

5. Main reinforcement runs across both the spans to resist two way bending