Acutance
The abruptness of the transition at an edge, which the eye reads as sharpness independently of true resolution.
In depth
Resolution is how much detail is present; acutance is how crisply the boundaries between tones are drawn. Unsharp masking works entirely by raising acutance — it invents no new detail, it simply steepens edges. This is why a modestly resolving image with high acutance often looks sharper than a high-resolution image with soft transitions.
Examples in practice
Two prints from the same file, one sharpened and one not, resolve identical detail but the sharpened one looks distinctly crisper.