Merriam-Webster Collegiate Dictionary - epoch

 noun  Etymology: Medieval Latin ~a, from Greek ~e cessation, fixed point, from epechein to pause, hold back, from epi- + echein to hold — more at scheme  Date: 1614  1.  a. an event or a time marked by an event that begins a new period or development  b. a memorable event or date  2.  a. an extended period of time usually characterized by a distinctive development or by a memorable series of events  b. a division of geologic time less than a period and greater than an age  3. an instant of time or a date selected as a point of reference (as in astronomy)  Synonyms: see period