era

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A period of time as reckoned from a specific date serving as the basis of its chronological system.
• noun A period of time characterized by particular circumstances, events, or personages.
• noun The longest division of geologic time, made up of one or more periods.

from The Century Dictionary.

• noun In geology, a division of geologic time which, according to the recommendation of the International Congress of Geologists, is to be regarded as of highest rank, corresponding to the stratigraphic term group. See group, 3 .
• noun A tale or count of years from a fixed epoch; a period during which, in some part or parts of the world, years are numbered and dates are reckoned from a particular point of time in the past, generally determined by some historical event. See phrases below.
• noun A series of years having some distinctive historical character: as, the era of good feeling (see below).
• noun Loosely, an epoch from which time is reckoned, or a point of time noted for some event or occurrence; an epoch in general: as, the era of Christ's appearance.
• noun A Cæsarean era beginning 48 b. c., Oct. 1st.
• noun An era coinciding with the reformed era of Alexandria.

from the GNU version of the Collaborative International Dictionary of English.

• noun A fixed point of time, usually an epoch, from which a series of years is reckoned.
• noun A period of time reckoned from some particular date or epoch; a succession of years dating from some important event
• noun A period of time in which a new order of things prevails; a signal stage of history; an epoch.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A time period of indeterminate length, generally more than one year.
• noun geology A unit of time, smaller than aeons and greater than periods.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a major division of geological time; an era is usually divided into two or more periods
• noun a period marked by distinctive character or reckoned from a fixed point or event
• noun (baseball) a measure of a pitcher's effectiveness; calculated as the average number of earned runs allowed by the pitcher for every nine innings pitched