1. [n]  a major division of geological time; an era is usually divided into two or more periods
2. [n]  a period marked by distinctive character or reckoned from a fixed point or event

Era
Era

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1. \E"ra\, n.; pl. {Eras}. [LL. aera an era, in earlier usage,
   the items of an account, counters, pl. of aes, aeris, brass,
   money. See {Ore}.]
   1. A fixed point of time, usually an epoch, from which a
      series of years is reckoned.
            The foundation of Solomon's temple is conjectured by
            Ideler to have been an era.           --R. S. Poole.
   2. A period of time reckoned from some particular date or
      epoch; a succession of years dating from some important
      event; as, the era of Alexander; the era of Christ, or the
      Christian era (see under {Christian}).
            The first century of our era.         --M. Arnold.
   3. A period of time in which a new order of things prevails;
      a signal stage of history; an epoch.
            Painting may truly be said to have opened the new
            era of culture.                       --J. A.
                                                  Symonds.
   Syn: Epoch; time; date; period; age; dispensation. See
        {Epoch}.
   

2. \
   The era in use in all Christian countries, which was intended
   to commence with the birth of Christ. The era as now
   established was first used by Dionysius Exiguus (died about
   540), who placed the birth of Christ on the 25th of December
   in the year of Rome 754, which year he counted as 1 a. d.
   This date for Christ's birth is now generally thought to be
   about four years too late.
   

1. Entity-Relationship-Attribute

2. Synonym epoch. Webster's Unabridged makes these words almost synonymous, but "era" usually connotes a span of time rather than a point in time.