olemus
(2a) võnkekõvera tippe läbiv joon
Wiktionary:
5. (electronics) a curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics
https://en.wikipedia.org/wiki/Envelope_(waves)

(2b) kõverate pere puutejoon või pindade pere puutepind
Wiktionary:
4. (geometry) a mathematical curve, surface, or higher-dimensional object that is the tangent to a given family of lines, curves, surfaces, or higher-dimensional objects
https://en.wikipedia.org/wiki/Envelope_(mathematics)
https://duckduckgo.com/?q=envelope+curve&t=h_&iax=images&ia=images