As an adjective achronal
is describing a set of points in a spacetime, no two of which have timelike separation.
As a noun achronality is
the condition of being achronal.
achronal
English
Adjective
(-)
(physics) Describing a set of points in a spacetime, no two of which have timelike separation achronality
English
Noun
(-)
(mathematics, physics) The condition of being achronal.