History: What is chorus?

What is chorus?

• Chorus makes the sound source appear to be a single instrument playing the same part instead of a group of instruments.
  The opposite is true.
• Chorus makes the sound source appear to be modulating frequency.
  That is closer to vibrato.
• Chorus makes the sound source appear to be a group of instruments playing the same part instead of a single instrument.
  Yes!
• Chorus makes the sound source appear to be modulating amplitude.
  That is closer to tremolo.