Merriam-Webster Collegiate Dictionary - era

 noun  Etymology: Late Latin a~, from Latin, counters, plural of aer-, aes copper, money — more at ore  Date: 1615  1.  a. a fixed point in time from which a series of years is reckoned  b. a memorable or important date or event; especially one that begins a new period in the history of a person or thing  2. a system of chronological notation computed from a given date as basis  3.  a. a period identified by some prominent figure or characteristic feature the ~ of the horse and buggy  b. a stage in development (as of a person or thing)  c. a large division of geologic time usually shorter than an eon Paleozoic ~  Synonyms: see period ERA  abbreviation  1. earned run average  2. Equal Rights Amendment