Resonance requires old amplitude to add constructively with new drive

Evidence

A structure or region (the “system”) is resonant with a source at frequency f if the system confines (reflects, returns, echoes, and so on) “old” drive amplitude so as to add (in aggregate) constructively with “new” or arriving drive amplitude, enhancing the power supplied by the drive to the system. — Eric J. Heller, Why You Hear What You Hear- An…, 905

Categorization

• DEFINE: Core definition of resonance framed in amplitude-storage terms — worth keeping as the base definition.