Dimension of R*C is time (i.e., s).
So, 1/(RC) has dimension if 1/s (i.e. Hz or cycles/s) or rad/s.
Conversion from Hz to rad/s is done by the dimensionless constant 2*pi, as well as conversion from cycles to radians.
Regards
The dimension of R*C is seconds (s).
The invers expression 1/RC is identical to the "angular frequency w" with w=2*Pi*f.
Since the frequency f is given in Hz=1/s, we get the angular frequency by multiplying it with 2*Pi (rad).
Hence, w is given in rad/s .