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 (sī′məl-tā′nē-əs, sĭm′əl-)
1. Happening, existing, or done at the same time. See Synonyms at contemporary.
2. Mathematics Containing variables for which there are values that can satisfy all the equations: simultaneous equations.

[Latin simul, at the same time; see sem- in Indo-European roots + English -taneous (as in instantaneous).]

si′mul·ta′ne·ous·ly adv.
si′mul·ta′ne·ous·ness, si′mul·ta·ne′i·ty (-tə-nē′ĭ-tē, -nā′-) n.
ThesaurusAntonymsRelated WordsSynonymsLegend:
Noun1.simultaneousness - happening or existing or done at the same time
timing - the time when something happens
conjunction, co-occurrence, coincidence, concurrence - the temporal property of two things happening at the same time; "the interval determining the coincidence gate is adjustable"
References in classic literature ?
It is worse; for you cannot sit motionless in the heart of these perils, because the boat is rocking like a cradle, and you are pitched one way and the other, without the slightest warning; and only by a certain self-adjusting buoyancy and simultaneousness of volition and action, can you escape being made a Mazeppa of, and run away with where the all-seeing sun himself could never pierce you out.
Soteriou and Chase (1998) mention that services, beyond their intangible and highly heterogeneous nature, are characterized by the simultaneousness between production and consumption, and consequently by the direct influence of customers (in the condition of coproducers) on the quality of provided services.
Influential contemporary Irish poet Derek Mahon also underscores Durcan's appropriation of 'the simultaneousness of disparate experience' as a cubist instead of a surrealist.
An Unbearable Longing strives to explore simultaneousness, juxtaposition, and the nostalgic and cultural implications of memory.