Saturation:

When I_B continues to rise, a point is reached ultimately where I_C rise less rapidly. Eventually, the I_C vs. I_B function, or characteristic curve, of the transistor levels off. The graph in figure below shows a family of characteristic curves for a theoretical bipolar transistor. The real current values depend on the specific type of device; values are larger for power transistors and smaller for weak-signal transistors. Wherever the curves level off, the transistor is in a condition of saturation. Under these situations, the transistor loses its ability to proficiently amplify signals. Though, the transistor can still work for switching purposes.

Figure: The family of characteristic curves for a hypothetical npn bipolar transistor.