a treatment which emulates the effect of two performers generating the same sound event. 

it usually means the copy of the original signal will be detuned slightly and placed slightly out of phase. 

If a delay is set to a short range (approximately 15-35 milliseconds long) and mixed with the dry signal, the effect is to simulate a second part doubling the dry signal. 
If the two signals, dry and delayed, are panned apart left and right a simple stereo enhancement and much bigger image can be achieved. 

Sound Examples:
  clarinet (mono)    clarinet doubled (stereo) 
See Also:
 Chorusing   Digital Delay   flanging