Clarity, Resolution, Sharpness, Contrast, and Acutance
Clarity in a photograph is a combination of resolution and acutance.

Resolution is the ability of the camera to distinguish between two very close elements in a photo.

Acutance is the edge contrast of an image.

Once you’ve opened the shutter and made a photograph there is no ability to change the resolution. Resolution is governed by your choice of camera - not all digital sensors are equal - and the settings of aperture and shutter speed. Technique comes into it as well: camera shake is obviously going to have a bad effect. A dirty lens won’t help. Shooting into the light and allowing flare will make it look like you shot through a veil.

Acutance is the only aspect of sharpness which is still under your control after the images have been made.

When processing a photograph you have an opportunity to improve acutance by using sharpening tools. Thus the edges of features become better defined and contrast is improved. An image appears to be sharper when edge contast differences are strong.

So that’s all clear, then.