epoch (n.) Look up epoch at Dictionary.com
1610s, epocha, "point marking the start of a new period in time" (such as the founding of Rome, the birth of Christ, the Hegira), from Late Latin epocha, from Greek epokhe "stoppage, fixed point of time," from epekhein "to pause, take up a position," from epi "on" (see epi-) + ekhein "to hold" (see scheme (n.)). Transferred sense of "a period of time" is 1620s; geological usage (not a precise measurement) is from 1802.