An infinitesimal or very short space of time a moment they arrived not an instant too soon. The point of time now present or present with reference to some action or event.
He climbed into bed and at that instant the telephone rang he came the instant (that) he heard the news.
Instant | Definition of Instant at Dictionary.com
Instant definition is - an infinitesimal space of time especially a point in time separating two states.
A very short moment of time, or a particular point in time at that instant, someone knocked on the door. Call me the instant you get home ( as soon as you get to your home).