'Envelope' comes from flight engineering. It describes the range of values and situations that would result in stable flight. If a pilot 'pushed the envelope', it referred to pushing the airplane to its limits. In engineering in general, a design envelope would be the range of forces that a structure would be able to withstand.

Answering questions on italki can really be some sort of imagination-training sometimes. How about some context? It might refers to some specific term in physics, or so.