simultaneously

Simultaneously is an adverb meaning at the same time; occurring or existing at one moment in time or within a shared time frame. The related adjective simultaneous describes something that happens at the same time; the adverb form is formed by adding -ly. Etymology traces to Latin simul meaning together and -taneus, with the form evolving through French and English.

In ordinary language, the word is used to indicate coincidence of events or actions, as in “The fireworks and the light show happened simultaneously.” In mathematics, simultaneous appears in the phrase simultaneous equations—systems of equations solved together for multiple variables. In science and technology, the term appears in contexts such as simultaneous computation or parallel processing, where several tasks are executed at the same time, or in synchronization, where timing relationships are maintained across components. In physics, the concept of simultaneity is central to discussions of time and measurement; in Einsteinian relativity, whether two events are simultaneous depends on the observer's frame of reference.

In music, simultaneous notes or chords are those sounding at the same moment. In linguistics and everyday speech, simultaneous action may be described as occurring together, often with verbs used in parallel.

See also synchronization, concurrency, clock synchronization, event, and reference frames.