ep·och 
(ĕpək, ēpŏk′)
(ĕpək, ēpŏk′)n.
1. A particular period of history, especially one considered remarkable or noteworthy.
2. A unit of geologic time that is a division of a period.
3. Astronomy An instant in time that is arbitrarily selected as a point of reference for specification of celestial coordinates.
[Medieval Latin epocha, measure of time, from Greek epokhē, a point in time; see segh- in the Appendix of Indo-European roots.]