Word of the Day: Envelope

en-ve-lope / ĕnvə-lōp, ŏn-və-lōp
1. a folded paper cover like a pocket for a letter, usually having a method of closure
Genius can write on the back of old envelopes but mere talent requires the finest stationery available.
Dorothy Parker, 1893 – 1967
2. a covering or something that wraps around an item
We are blessed with a place that is open to the universe and, despite this, supports this very thin envelope of air we call atmosphere, which holds just the right amount of oxygen for us to breathe.
Sylvia Earle, 1935 –
3. in biology, a structure that encloses another, such as a membrane or skin
In the 1950s and 1960s, as surgeons adopted the principles of anatomic restoration of the articular surface, stable fixation of fractures, and early motion, concern for the soft tissue envelope was secondary to anatomic reduction and rigid fixation.
David A. Volgas, “Care of the Soft Tissue Envelope”, www.musculoskeletalkey.com/care-of-the-soft-tissue-envelope, accessed July 27, 2021
4. the membrane enclosing the gasbag of an aircraft such as a balloon or dirigible, or the gasbag itself
Non-rigid dirigibles, often called blimps, rely on the gas pressure in the envelope to maintain their shape. 
“Controllable Balloons: Dirigibles”, ‘National Museum of the United States Airforce’, www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/197535/controllable-balloons-dirigibles, April 7, 2015
5. in mathematics, a curve or surface that is tangent to all of the curves or surfaces in a set
Two parallel lines are the envelope of the family of circles of the same radius having centres on a straight line. 
Britannica, The Editors of Encyclopaedia. “Envelope”. Encyclopedia Britannica, 3 Feb. 2009, www.britannica.com/science/envelope-mathematics, accessed 27 July 2021
6. the sealed glass or housing of a vacuum tube
Vacuum tubes then must have a sort of hermetic envelope, carefully evacuated and sealed after the electrodes have been assembled.
“Introduction to vacuum tubes”, www.ase-museoedelpro.org/Museo_Edelpro/Catalogo/Introduction%20to%20vacuum%20tubes.pdf, accessed July 27, 2021
7. a set of limitations that is conventionally accepted
Keep pushing the envelope, and remember that every dream starts off small. 
Robin Sharma, 1964 –
Envelope comes through the Middle English verb envolupen (enclose, cover) and the Middle French verb envoluper/enveloper of the same meaning from the Old French verb envoloper, also of the same meaning, a combination of the French prepositional prefix en- (in) and the verb voloper (wrap).
Thank you to Allen Ward for providing this etymology.