In physicslang=en terms the difference between spacetime and achronal
is that
spacetime is (physics) an
n''dimensional continuum consisting of dimensions of both space & time normally spacetime is considered as having 4 dimensions (''x'', ''y'', ''z'', ''t ), but higherdimensional spacetimes are often encountered in theoretical physics, eg the 5dimensional spacetime of kaluzaklein theory or the 11 dimensions of spacetime in mtheory while
achronal is (physics) describing a set of points in a spacetime, no two of which have timelike separation.
As a noun spacetime
is (uncountablephysics) the fourdimensional continuum of the three spatial dimensions plus time.
As an adjective achronal is
(physics) describing a set of points in a spacetime, no two of which have timelike separation.
Other Comparisons: What's the difference?
spacetime Noun
(uncountable, physics) The fourdimensional continuum of the three spatial dimensions plus time.
 An event is a point in spacetime , specified by the coordinates x,y,z and t.
(physics) An n''dimensional continuum consisting of dimensions of both space & time. Normally spacetime is considered as having 4 dimensions (''x'', ''y'', ''z'', ''t ), but higherdimensional spacetimes are often encountered in theoretical physics, e.g. the 5dimensional spacetime of KaluzaKlein theory or the 11 dimensions of spacetime in Mtheory.
(relativity) A specific region of the universe with mathematically different properties than the surrounding spacetime. Synonymous with "metric" within the context of general relativity.
 "a Schwarzschild spacetime," "a ReissnerNordstrÃ¶m spacetime," etc. as opposed to sense (2) describing the universe's spacetime as a whole: "a Minkowski spacetime," "a 5dimensional spacetime," etc.
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achronal English
Adjective
()
(physics) Describing a set of points in a spacetime, no two of which have timelike separation 