Error is a measurement of the deviation of an interval from another interval that it is approximating.

For example, the tempered ~3/2 of quarter-comma meantone is 696.578¢, while a just 3/2 is 701.955¢, so the error of this ~3/2 is 696.578¢ - 701.955¢ = -5.377¢.

In Paul Erlich's Middle Path paper, error is called mistuning.

See also