It's not guesswork to say that, in general,  the "theory of ether" is undefined.   Newton's bucket argument probably best identifies the requirement for an "ether."  The quantity that we refer to as direction does meet this requirement.

13 minutes ago, steveupson said:

The quantity that we refer to as direction does meet this requirement.

Stop trying to drag every thread into a discussion of your pet "theory". (Reported for hijacking.)

Ok then, what is the definition?

This discussion is about science.  Why do y'all insist on making it about me?  It isn't my fault that the universe is the way it is.

14 minutes ago, steveupson said:

Ok then, what is the definition?

The [luminiferous] was a proposed medium for the transmission of light. It was later found to be (1) not necessary, (2) physically implausible and (3) undetectable.

Nothing to do with wacky ideas about "direction".