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 I, too, have often pondered over how to define time.  I also often experience some frustration when I realize that the second that just passed will be one that I can never live through again.  That time is gone, and the change that happened through out it cannot ever be undone.  We often treat time like it is almost a solid entity, perhaps even permeable.  The idea of time-travel is harder and harder for me to imagine every time I think about it.  If one were to travel in time, who knows what could change, in other words, how realistic is the Butterfly Effect?  If I were to go back in time and redo my life, say, go to a different college or choose a different major, would something so seemingly insignificant have an impact beyond myself?  I think perhaps time is simply the continual change the universe goes through.  Time is also infinite.  Take a minute, for example.  It can be divided into seconds, which can then be divided into milliseconds and so on.  In a way, we have already lived an infinite number of moments.