[proper noun](generally) The basis point for measuring years in a calendar; the year 0 for any given time-reckoning system.
[proper noun](historical) The era of all emperors to have ever ruled Japan, starting with Emperor Jinmu's supposed ascension in 660 BCE, established in 1872 (5th year of the Meiji era).
(This term, きげん(kigen), is the hiragana spelling of the above terms.)