Simultaneity is the relation between two events assumed to be happening at the same time in a given 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 relative to one another at low speeds compared to the speed of light, this effect is small and can for practical matters be ignored, allowing simultaneity to be treated as an absolute relation.
The word is derived from the Latin simul, meaning 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.
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