History: What is the decay phase? How do we measure it?

What is the decay phase? How do we measure it?

• The sound returns the amplitude back down to zero. Time.
  That is release.
• The sound holds that amplitude until release. Relative amplitude.
  That is sustain.
• The sound ramps back down to a somewhat lower amplitude level. Time.
  Great work!
• The sound ramps up from zero amplitude to its highest amplitude. Time.
  That is attack.