When I began working at my uncle's studio in 1975, engineer Vance Frost, who was a part time mentor, insisted that "echo" was singular and "reverb" was plural.
In other words, an echo was a single slap or reflection, and reverb was an aggregate of reflections. I'm not sure how many in the industry would agree, but it's always made sense to me.
Terry, thanks for the kind words. I showed your comments to my wife, and they gave her a good laugh.