# Simultaneity

Simultaneity is the property of two events happening at the same time in a frame of reference. According to Einstein's Theory of Relativity, simultaneity is not an absolute property 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 at speeds small compared to the speed of light with respect to one another this effect is small and can for practical matters be ignored such that simultaneity can be treated as an absolute property.

The word derives from the Latin simul, at the same time (see sem-1 in Indo-European Roots) plus the suffix -taneous, abstracted from spontaneous[1] (which in turn comes directly from Latin[2]).

The noun simult means a supernatural coincidence, two or more divinely inspired events that occur at or near the same period of time that are related to each other in both noticeable and unnoticeable characteristics.[citation needed]

• In econometrics, it arises when one or more of the explanatory variables is jointly determined with the dependent variable, typically through an equilibrium mechanism.
• In criminal law, for a criminal violation to be established, it usually must be shown that there was simultaneity of actus reus and mens rea.
• In mathematics, a system of equations or a set of simultaneous equations share variables; a solution is a set of variable values for which all these equations are satisfied together.
• In music, see Simultaneity (music).
• In physics, see Relativity of simultaneity.
• In marketing, simultaneity is one of the characteristics of a service which differentiates it from a product. It refers to the idea that the production and consumption of a service occur simultaneously, making it impossible to produce and store a service prior to consumption.
• Simultaneous publishing is the publication of a work by two or more entities within a short duration, requiring both entities to hold non-exclusive rights to the work.