The three components of "sharpness" are resolution, contrast and accutance. Accutance refers to the degree of visibility of the edges of details. You are correct about the effect of the "sharpening" function in Photoshop et al.
There are some excellent observations about "sharpness" in the early parts of Ctein's "Post Exposure" which is now available on the web: http://ctein.com/booksmpl.htm

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