Like many things in D&D or gaming in general, the answer is 'all of the above'. No, most scientists, unless they are also genre readers or gamers, won't know what that means save perhaps in a general sense. They might use the term 'dimension' but that's not right, either, since 'dimensions' are not 'other worlds'. 'Parallel Universe' might be closest.
Most religions have the concept of 'some place that isn't this one, that is normally imperceptable to human senses but occassionally people can go there and sometimes come back again'. That'd probably be the basis for the entire idea of 'other planes'.
Right off the top of my head, the first place I can think of that refers to 'planes' is Theosophical writings (which itself is a kind of bizarre marriage of religious and scientific ideas), though that probably pulls in things from previous occult traditions (since most occult traditions cross-pollinate). Most occult thinking of that time refers to an 'astral plane', which Theosophy seems to have invented - the dictionary refers to 'astral' as
1. Of, relating to, emanating from, or resembling the stars.
2. Biology Of, relating to, or shaped like the mitotic aster; star-shaped.
3. Of or relating to a supersensible body believed by theosophists to coexist with and survive the death of the human physical body
Ethereal, among it's other definitions, has - a. Of the celestial spheres; heavenly. b. Not of this world; spiritual.
Ether, if I remember right, comes from Huygens, a Dutch philosopher who put forth the idea of an all-pervading medium which would explain why heat and light could travel in a vacuum.
D&D takes those ideas and blends them all together. In most occult circles, 'astral' and 'ethereal' are used interchangably; the astral body can also be referred to as the 'ethric body'. The 'silver cord' idea in the Astral Plane is taken directly from Theosophical teachings (or, probably more accurately, popular fiction using Theosophical ideas and terms; the terms have become so common in fiction that it's probably impossible to pinpoint an exact source unless Col_Playdoh has a really fantastic memory).
THERE has been an infinite confusion of names to express one and the same thing.
The chaos of the ancients; the Zoroastrian sacred fire, or the Antusbyrum of the Parsees; the Hermes-fire; the Elmes-fire of the ancient Germans; the lightning of Cybele; the burning torch of Apollo; the flame on the altar of Pan; the inextinguishable fire in the temple on the Acropolis, and in that of Vesta; the fire-flame of Pluto's helm; the brilliant sparks on the hats of the Dioscuri, on the Gorgon head, the helm of Pallas, and the staff of Mercury; the [[pur asbeston]];the Egyptian Phtha, or Ra; the Grecian Zeus Cataibates (the descending);* the pentecostal fire-tongues; the burning bush of Moses; the pillar of fire of the Exodus, and the "burning lamp" of Abram; the eternal fire of the "bottomless pit"; the Delphic oracular vapors; the Sidereal light of the Rosicrucians; the AKASA of the Hindu adepts; the Astral light of Eliphas Levi; the nerve-aura and the fluid of the magnetists; the od of Reichenbach; the fire-globe, or meteor-cat of Babinet; the Psychod and ectenic force of Thury; the psychic force of Sergeant Cox and Mr. Crookes; the atmospheric magnetism of some naturalists; galvanism; and finally, electricity, are but various names for many different manifestations, or effects of the same mysterious, all-pervading cause -- the Greek Archeus, or [[Archaios]].
That's from the astral light chapter of
Isis Unveiled, Madame Blavatsky's first (?) work. That should give you terminology to mine for game effect for a few weeks
The whole thing can be found here.
