An instant is an infinitesimal moment in time, a moment whose passage is instantaneous.
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 seconds. Within the framework of the laws of physics as they are understood today, for times less than one Planck time apart, we can neither measure nor detect any change. As of May 2010, the smallest time interval that was directly measured was on the order of 12 attoseconds (12 × 10 seconds), about 10 times larger than the Planck time. 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.