Why is it called the ‘Sound Barrier’? Does it separate two thing like sound and time?

Question: Why is it called the 'Sound Barrier'? Does it separate two thing like sound and time?
- Keywords:
  sound,
  sound barrier,
  time
Asked by lucario13 to Dr D on 23 Jun 2013.
- Andrew Devitt answered on 23 Jun 2013:
  
  Hi @lucario13
  I’m no expert on this but my understanding is that the sound barrier is that point when you change from moving below the sped of sound to above the speed of sound. So the barrier separates those two states.

Question: Why is it called the 'Sound Barrier'? Does it separate two thing like sound and time?