n.
Variant: or eon
Pronunciation: ' ē - ə n, ' ē - ˌ än
Function: noun
Etymology: Latin, from Greek ai ō n ― more at AYE
Date: 1647
1 : an immeasurably or indefinitely long period of time : AGE
2 a usually eon : a very large division of geologic time usually longer than an era b : a unit of geologic time equal to one billion years