Acutance
From The Glossary of Digital Photography
The apparent sharpness caused by the amount of edge contrast between two adjoining elements in an image. The higher the edge contrast, the more apparent sharpness. This is due to the way that human vision operates. The higher apparent sharpness appears to increase resolution, but there is no change in the actual resolution of an image. This is the principle used by Photoshop in the unsharp masking process. See also unsharp masking (USM)
