Glossary of Terms
 
  velocity sensitivity  
  1) The ability of a MIDI controller (often a MIDI keyboard) to change the amount of a MIDI parameter in a sound by playing hard or soft. This type of touch sensitivity is commonly used with piano sounds to create realistic dynamics.
2) The ability of a MIDI synthesizer or tone generator to respond to MIDI velocity messages.
 
  Source: Yamaha  

Also refered as:
  initial touch   touch sensitivity
Related terms:
  MIDI   MIDI controller   aftertouch