In remote sensing, ground sample distance (GSD) refers to the dimensions of a single pixel in an image as measured on the ground. The [calculation][1] of GSD uses the focal length and sensor array dimensions, which are properties of the sensor, as well as the distance between the sensor and the ground *at the moment of image capture*, normally represented by the altitude of an aircraft or orbital distance of a satellite. 

Because some objects can stand quite high above the ground relative to the altitude of a flying craft (i.e., buildings or trees), because images can cover very large areas, and due to variable surface relief, the actual resolution of objects across an image can differ from a correctly calculated GSD.

So in short, GSD is a standardized way to describe the resolution of remotely sensed aerial imagery, while [resolution][2] without georeferencing typically refers to solely to the number of pixels in the sensor array array, which is the product of the height and width and usually expressed in megapixels (i.e., 8192 × 5460 = ~45MP). The former describes the size of the smallest detectable object on the ground within a particular image, while the latter describes the level of detail recorded by a particular sensor.


  [1]: https://support.pix4d.com/hc/en-us/articles/202559809-Ground-sampling-distance-GSD-in-photogrammetry
  [2]: https://en.wikipedia.org/wiki/Image_resolution