An Etymological Dictionary of Astronomy and Astrophysics

1. General: Happening, existing, or operating at the same time.
   

2. In a → space-time diagram, the points that have the same time values. Two events that are simultaneous in one → inertial frame are not, in general, simultaneous in another inertial reference frame moving relative to the first. → relativity of simultaneity. Compare → synchronous.

Etymology (EN): From L.L. simultaneus, from L. simul “at the same time” + -taneous, abstracted from → spontaneous.

Etymology (PE): Hamzamân, from ham-, → syn- + zamân, → time.