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Simultaneity is the relation between two events assumed to be happening at the same time in a frame of reference. According to Einstein's theory of relativity, simultaneity is not an absolute relation between events; what is simultaneous in one frame of reference will not necessarily be simultaneous in another. (See Relativity of simultaneity.) For inertial frames moving with respect to one another at speeds small compared to the speed of light, this effect is small and can for practical matters be ignored, such that simultaneity can be treated as an absolute relation.
The word derives from the Latin simul, at the same time (see sem-1 in Indo-European Roots) plus the suffix -taneous, abstracted from spontaneous (which in turn comes directly from Latin). The noun simult means a supernatural coincidence, two or more divinely inspired events that occur at or near the same period of time that are related to each other in both noticeable and unnoticeable characteristics.