n.
1.the earliest eon in the history of the Earth from the first accretion of planetary material (around 4,600 million years ago) until the date of the oldest known rocks (about 3,800 million years ago)
Idiom of the Day
sleep on (something)
to think about something, to consider something, to decide something later
"I will sleep on the proposal tonight and I will give you an answer tomorrow."