What is Ager?

Legal Definition
Lat. In civil law. A field; land generally. A portion of land enclosed by definite baundaries. Municipality No. 2 v. Orleans Co.tton Press, 18 La. 167, 36 Am. Dec. 624. In old English law. An acre. Spelman.
-- Black's Law Dictionary
Legal Definition
A field.
-- Ballentine's Law Dictionary