- For other uses of the term ether (and aether), see the fork page ether
Ether, or æther, was
the stuff used to replenish mana/MP Ike's signature skill in Fire Emblem an invisible substance that was theorized to fill all the problem parts in early (pre-Einstein) physics theories.
In its final incarnation, it was invoked to fill all of outer space, thus explaining how light could travel its expanses with no medium to carry it.
 A brief history of ether
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Ether had a long and respected history in physics. It generally played the role of That Thing Which Magically Solves All the Problems in My Pet Theory, a role currently enjoyed by dark energy.
The idea of an ether-like substance in physics has been around since Aristotle, whose essay in Physics Book IV, section 8 was eventually summarized over the years as the statement: "Nature abhors a vacuum". This concept led philosophers to speculate that all the empty space in the world must be filled with something, even when they didn't appear to be. That thing was, of course, ether.
Since mechanical waves need a medium, it was concluded that light and radio waves must need a medium, the ether, as well. The idea was upheld until the late nineteenth century when the Michelson-Morley experiment disproved its existence.