History: What is an equalizer?

What is an equalizer?

• An equalizer is a sound modifying module that helps to balance the frequencies of the incoming signal.
  Perfect!
• An equalizer is a sound modifying module that helps to balance the phases of the incoming signal.
  Nope.
• An equalizer is a sound modifying module that helps to balance the duration of the incoming signal.
  Nope.
• An equalizer is a sound modifying module that helps to balance the amplitudes of the incoming signal.
  There's a better answer.