The omer (Hebrew: עמר) is an ancient Israelite unit of dry measure used in the era of the Temple in Jerusalem. It is used in the Bible as an ancient unit of volume for grains and dry commodities, and the Torah mentions as being equal to one tenth of an ephah. The ephah was defined as being 72 logs, and the log was equal to the Sumerian mina, which was itself defined as one sixtieth of a maris; the omer was thus equal to about 12⁄100 of a maris. The maris was defined as being the quantity of water equal in weight to a light royal talent, and was thus equal to about 30.3 litres, making the omer equal to about 3.64 litres.
The omer is not easy to fit into the remainder of the ancient Israelite measurement system, as it constitutes 1.8 kabs and 0.3 se'ah. It is alleged that the unit is a result of the sexagesimal system being decimalised, perhaps under the influence of Egypt or Assyria, which both had decimal systems.
The word omer is sometimes translated as sheaf — specifically, an amount of grain large enough to require bundling. The biblical episode of the manna describes God as instructing the Israelites to collect an omer for each person in your tent, implying that each person could eat an omer of manna a day. In the Instructions of Moses (Torah in Hebrew), the main significance of the omer is the traditional offering of an omer of barley on the day after the Sabbath during the feast of unleavened bread (during the period of Temple sacrifice) as well as the tradition of the Counting of the Omer (sefirat ha'omer) - the 49 days between this sacrifice and the two loaves of wheat offered on the holiday of Shavuot.
The omer should not be confused with the homer, a much larger unit of volume for liquids.