simultaneous

adjective

si·​mul·​ta·​neous ˌsī-məl-ˈtā-nē-əs 

-nyəs

also ˌsi-

1

: existing or occurring at the same time : exactly coincident

2

: satisfied by the same values of the variables

simultaneous equations

simultaneity

ˌsī-məl-tə-ˈnē-ə-tē  -ˈnā- 

also ˌsi-

noun

simultaneously

ˌsī-məl-ˈtā-nē-əs-lē 

-nyəs-

also ˌsi-

adverb

simultaneousness noun

Synonyms of simultaneous

• concurrent
• synchronous
• synchronic
• coincident

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

Breaking that cycle will require simultaneous action across multiple fronts, including stronger policy frameworks, financial incentives and clearer demand signals from brands, such as commitments to incorporate recycled fibers into products. —Rhonda Richford, Footwear News, 23 Mar. 2026

In a process called simultaneous contrast, our brain perceives colored objects in relation to the color of the background that they’re viewed against. —Nora Bradford, Scientific American, 23 Mar. 2026

The arson and assaults in four Nablus-area villages Sunday came a day after the official Palestinian news agency WAFA reported simultaneous attacks in at least six communities overnight — the latest spate of violence in the West Bank. —Sam Metz, Los Angeles Times, 23 Mar. 2026

The arson and assaults in four Nablus-area villages yesterday came a day after the official Palestinian news agency WAFA reported simultaneous attacks in at least six communities overnight — the latest spate of violence in the occupied West Bank. —Marlene Lenthang, NBC news, 23 Mar. 2026

Word History

Etymology

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

Rhymes for simultaneous

• miscellaneous
• percutaneous
• subcutaneous
• subterraneous
• transcutaneous
• contemporaneous
• extemporaneous
• intracutaneous
• mucocutaneous
• extraneous
• spontaneous
• lanius

Kids Definition

simultaneous

adjective

si·​mul·​ta·​neous ˌsī-məl-ˈtā-nē-əs 

-nyəs

1

: existing or occurring at the same time

2

: satisfied by the same values of the variables

simultaneous equations

simultaneously adverb

Etymology

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