An omer is a Biblical unit of measurement roughly equal to 2 quarts or 2 liters.

The omer represents one day's ration and is identified as one 1/10 ephah.

Omer only appears once in the Bible and is used in Exodus 16:22 as the amount of manna that was collected by the Israelites in one day.

Closely related to the omer is the word issaron which means 1/10.  It is used several times in the Exodus, Leviticus, and Numbers.