a major division of geological time; an era is usually divided into two or more periods
"Paleontologists often classify the Mesozoic era as being divided into three distinct periods, including the Jurassic and Cretaceous."
(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
"The rookie ace posted an ERA of just 2.15 last season, signaling his readiness to lead the rotation into the playoffs."
A time period of indeterminate length, generally more than one year.
"After three innings, the pitcher's ERA was already rising to an alarming 6.50."
Initialism of earned run average, a baseball statistic.
In plain English: An era is a long period of time marked by special events or changes.
"The industrial era changed how people lived and worked forever."
Usage: Use "era" to refer to a long and distinct period in history or the development of an institution. Do not use it as an abbreviation for earned run average, which is written as ERA.
Initialism of Equal Rights Amendment.
"The ERA is a critical piece of legislation that many activists are still working to have ratified by all states."
The word era was borrowed from Late Latin aera. It originally referred to a period of time marked by significant events or changes.