What is the difference between Torsional critical speed and critical speed of steam turbine?

critical speed is a overall spectrum...while lateral,bilateral, and torsional critical speed include in this spectrum.

Critical speed of the turbine is the rotor speed at which natural frequency of the assembled rotor (rotor shaft with discs, blades, shrouding strips etc in assembled condition) becomes equal to the operating speed. This is usually a expressed as a range (critical speed range).

critical speed may be angular(torsional) or lateral.