Excerpt from Wikipedia
Source: http://en.wikipedia.org/wiki/Present
Date: 28-Apr-2015
The present (or now) is the time that is associated with the events perceived directly and for the first time, not as a recollection (perceived more than once) or a speculation (predicted, hypothesis, uncertain). It is a period of time between the past and the future, and can vary in meaning from being an instant to a day or longer. In radiocarbon dating, the “present” is defined as AD 1950.
It is sometimes represented as a hyperplane in space-time, typically called “now”, although modern physics demonstrates that such a hyperplane cannot be defined uniquely for observers in relative motion.