General terms:
* Clarity
* Definition
* Detail
* Resolution (though this technically refers to the number of pixels, it's often used to describe how much detail is visible)
More technical terms:
* Acutance (describes the abruptness of edges in an image)
* Sharpness of detail
* Edge detail
* Spatial frequency response (describes how well the image sensor or lens captures different frequencies of detail)
The best term to use will depend on the specific context and what you want to emphasize.