What are the largest divisions of time?

We commonly use time units such as second, minute, hour, day, and year to describe the time. These scales are of relevance to us and our lives, since our life spans are short relative to how long Earth has existed. Scientists, on the other hand, use much larger divisions of time to describe events of scientific interest. Geologists especially use much larger time divisions, such as eons, epochs, era, periods, and ages.

Here are time divisions of interest to human beings, in increasing order:

seconds

minute (1 min = 60 seconds)

hour (1 hour = 60 minutes)

day (1 day = 24 hours)

year (1 year = about 365 days)

Decade (1 decade = 10 years)

century (1 century = 10 decades = 100 years)

Millennium (1 millennium = 10 centuries = 1000 years)

Age: a few million years

Epoch: a few million years

Period: a few to tens of million years 

Era: at least fifty million years up to a couple of hundred million years

Eon: spans over hundreds of millions of years

Thus, eons, eras, and periods are among the largest time divisions.

Hope this helps.