The name refers to the time constant an RC element must possess to attain the relevant frequency characteristic. 
 

Caption: RC circuits.

The time constant can be expressed as follows:
t  =     R × C
where 
t    =    Time constant [s]
R    =    Resistance [W]
C    =    Capacitance [F]

or: 
t    =    1 / 2×p×f₀    
where:
t   =     Time constant [s]
f₀    =     Corner frequency of the emphasis circuit (the 3 dB point).

The filter is a first-order type, i.e. the slope of the inclined part of the curve is 6 dB/octave.

A number of standardized time constant-based pre-/de-emphasis techniques are used, some in transmission techniques and others in tape recording. 
 Caption: The frequency responses of 50/75 µs (pre-)emphasis.

Some of the most frequently used time constants are listed here, along with the corresponding corner frequencies: