The ability to resolve different objects in the displayed CT image, when the difference in attenuation between the objects and the background is large compared to noise; normally a difference corresponding to at least one hundred HU is considered adequate.
Spatial resolution refers to the area on the ground that an imaging system, such as a satellite sensor, can distinguish. There are many measures of spatial resolution, the most common include the Instantaneous Field of View (IFOV), and the Effective Instantaneous Field of View (EIFOV).
Roughly speaking, the degree to which fine detail can be seen in an image. More precisely, resolution is the smallest distance between two objects that can be barely distinguished in the data. Note that if the resolution is limited by pixel size (which is the usual case in vegetation remote sensing), pixel size (in meters) is not the same as resolution. For example, suppose two bright, sub-pixel objects on a dark background are sampled in a square grid detector array. If one object occurs in the lower left corner of a pixel, and the second object occurs in the upper right corner of a pixel to the upper right of the first pixel, these two objects are not resolved (the data would still just show a two-pixel blob against a dark background). Any further apart, however, and they would be resolved. So if the pixel size is d×d meters, the scale of the finest object that can be resolved (r) in the image is actually given by r2=(2d)2+(2d)2, i.e. r~2.8d.
The smallest spatial detail in an image that can be resolved. Spatial resolution is described in a variety of ways, including the IFOV or the Airy disk size (both previously defined). As a broad generalization, spatial resolution is sometimes described in terms of the number of pixels in the imager array (angular FOV of the lens and the range to the target must also be considered).
Resolution is important in the ability to recognize and distinguish features. For raster data, the spatial resolution is the size of each grid-cell or pixel. For example, the VEMAP2 collection is made up of raster data for the USA at 0.5 degree latitude by longitude resolution. This means that the USA is divided into 0.5 degree grid-cells and each grid-cell is represented by a single data value. For vector data, the spatial resolution is the precision at which the location and shape of geographic features are stored. For example, a resolution of 1 meter means that the true ground position of a coordinate is within one meter of the stored coordinate values.