Which term defines a standard measure of volume, such as gallons or acre-feet?

The term that defines a standard measure of volume, such as gallons or acre-feet, is "volume." Volume is a quantifiable representation of the three-dimensional space an object occupies or the capacity of a container. In various fields, including water resource management and pesticide application, having a clear understanding of volume is crucial for proper dosing and application rates.

In contrast, a planimeter is a device used for measuring the area of a two-dimensional shape, not volume. Flow rate describes how much fluid passes a point in a given period, which relates to movement rather than the capacity itself. GPA, which stands for gallons per acre, refers to a pesticide application rate but is not a general term for measuring volume. Thus, the most accurate term from the provided choices for standard measures of volume is indeed "volume."