si·​mul·​ta·​neous ˌsī-məl-ˈtā-nē-əs How to pronounce simultaneous (audio)
 also  ˌsi-
: existing or occurring at the same time : exactly coincident
: satisfied by the same values of the variables
simultaneous equations
 also  ˌsi-
ˌsī-məl-ˈtā-nē-əs-lē How to pronounce simultaneous (audio)
 also  ˌsi-
simultaneousness noun
Choose the Right Synonym for simultaneous

contemporary, contemporaneous, coeval, synchronous, simultaneous, coincident mean existing or occurring at the same time.

contemporary is likely to apply to people and what relates to them.

Abraham Lincoln was contemporary with Charles Darwin

contemporaneous is more often applied to events than to people.

contemporaneous accounts of the kidnapping

coeval refers usually to periods, ages, eras, eons.

two stars thought to be coeval

synchronous implies exact correspondence in time and especially in periodic intervals.

synchronous timepieces

simultaneous implies correspondence in a moment of time.

the two shots were simultaneous

coincident is applied to events and may be used in order to avoid implication of causal relationship.

the end of World War II was coincident with a great vintage year

Examples of simultaneous in a Sentence

The two gunshots were simultaneous. a simultaneous release of the movie and its soundtrack on CD
Recent Examples on the Web Ramirez is the brother of Rogelio Angel Ramirez, 28, who was arrested in July and charged with accomplice to soliciting or recruiting a minor to join or to remain a member of a criminal gang, organization or enterprise and simultaneous possession of drugs and a firearm. Ron Wood, Arkansas Online, 19 Oct. 2023 With more consumers migrating to on-demand streaming video, advertisers are placing even more importance on big sports spectacles, which tend to draw the large simultaneous audiences Madison Avenue craves. Brian Steinberg, Variety, 18 Oct. 2023 And Hezbollah has engaged with Israel in attacks that are gradually claiming more lives on both sides and expanding deeper into each other’s territory – including simultaneous strikes Thursday against 19 Israeli targets. Scott Peterson, The Christian Science Monitor, 3 Nov. 2023 In addition to claiming a proclivity toward consumer privacy, Matic also stands out from other robot vacuums by relying on five RGB cameras and simultaneous localization and mapping (SLAM) for navigation, foregoing things like sensors and light detection and ranging (lidar). Scharon Harding, Ars Technica, 3 Nov. 2023 The navigation system does use simultaneous localization and mapping (SLAM) technology, found on most high-end vacuums, but Nariyawala says the Matic has 10 times better implementation thanks to its semantic understanding powered by software algorithms and data processing on the edge. Jennifer Pattison Tuohy, The Verge, 2 Nov. 2023 On September 14th, the U.A.W.’s new president, Shawn Fain, called the first simultaneous strike against all Big Three companies in the union’s history. Dan Kaufman, The New Yorker, 28 Oct. 2023 In a simultaneous wave of attacks on at least seven military posts across the border, Hamas sought to systematically disable key detection, communications and warning systems, using snipers and commercial drones armed with explosives. Loveday Morris, Washington Post, 27 Oct. 2023 The union had never engaged in a simultaneous strike against all three companies. Marley Jay, NBC News, 25 Oct. 2023 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'simultaneous.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


borrowed from Medieval Latin simultāneus "occurring at the same time," from Latin simul "in company, together, at the same time" (going back to an apocopated neuter form of similis "having characteristics in common, like") + Medieval Latin -tāneus, as in instantāneus instantaneous, Late Latin momentāneus "momentary," spontāneus "voluntary, unconstrained, spontaneous" — more at similar

Note: The Dictionary of Medieval Latin from British Sources records one occurrence of simultāneus, in the Tractatus de logica of John Wycliffe. At the entry in that dictionary the word is parsed as from simultās "occurrence at the same time" (distinct from classical Latin simultās "state of animosity") + the suffix -āneus. This analysis is possible but seems less likely. Regarding the suffix -āneus see the etymology and note at spontaneous.

First Known Use

circa 1660, in the meaning defined at sense 1

Time Traveler
The first known use of simultaneous was circa 1660

Dictionary Entries Near simultaneous

Cite this Entry

“Simultaneous.” Dictionary, Merriam-Webster, Accessed 29 Nov. 2023.

Kids Definition


si·​mul·​ta·​neous ˌsī-məl-ˈtā-nē-əs How to pronounce simultaneous (audio)
: existing or occurring at the same time
: satisfied by the same values of the variables
simultaneous equations
simultaneously adverb

from Latin simul "at the same time, together" and English -taneous (as in instantaneous) — related to assemble, ensemble

More from Merriam-Webster on simultaneous

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