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(uncountable, literary or poetic) The substance formerly supposed to fill the upper regions of the atmosphere above the clouds, in particular as a medium breathed by deities.
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(uncountable, physics, historical) Often as aether and more fully as luminiferous aether: The hypothetical substance permeating space, functioning as a medium for electromagnetic waves to propagate through, and which does not exert resistance to the movement of matter; its existence is incompatible with Einstein's theory of relativity; famously found to be undetectable by the 1887 Michelson–Morley experiment.
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(uncountable, colloquial) The atmosphere or space as a medium for broadcasting radio and television signals; also, a notional space through which Internet and other digital communications take place; cyberspace.
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(uncountable, colloquial) A particular quality created by or surrounding an object, person, or place; an atmosphere, an aura.
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(uncountable, organic chemistry) Diethyl ether (C₄H₁₀O), an organic compound with a sweet odour used in the past as an anaesthetic.
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(countable, organic chemistry) Any of a class of organic compounds containing an oxygen atom bonded to two hydrocarbon groups.
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(uncountable) Starting fluid.