1 : produced or occurring with or as if with extreme rapidity and ease. 2 : immediate, direct the play was an instant success. 3a(1) : premixed or precooked for easy final preparation instant pudding. (2) : appearing in or as if in ready-to-use form instant poetry. b : immediately soluble in water instant coffee.
In physics and the philosophy of science, instant refers to an infinitesimal interval in time, whose passage is instantaneous. In ordinary speech, an instant has been defined as "a point or very short space of time," a notion deriving from its etymological source, the Latin verb instare, from in- + stare ('to stand'), meaning 'to stand upon or near.'
The continuous nature of time and its infinite divisibility was addressed by Aristotle in his Physics, where he wrote on Zeno's paradoxes. The philosopher and mathematician Bertrand Russell was still seeking to define the exact nature of an instant thousands of years later.
In physics, a theoretical lower-bound unit of time called the Planck time has been proposed, that being the time required for light to travel a distance of 1 Planck length. The Planck time is theorized to be the smallest time measurement that will ever be possible, roughly 10−43 seconds. Within the framework of the laws of physics as they are understood today, for times less than one Planck time apart, one can neither measure nor detect any change. It is therefore physically impossible, with current technology, to determine if any action exists that causes a reaction in "an instant", rather than a reaction occurring after an interval of time too short to observe or measure.