This refers to the level of detail of the image, in two senses:
Radiometric: number of energy levels that can be distinguished. Can be practically infinite in photographs, but limited in digital images
Spatial: smallest object that can be distinguished in the image.
| This is the same
    geographic extent -  the University of Florida campus and central Gainesville shown with different resolutions, from lowest to highest resolution:  | 
  
| 1. Multispectral Scanner Image (MSS) - 60 meter resolution | 
| 2. Thematic Mapper Image (TM) - 30 meter resolution | 
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| 3. SPOT Image - 10 meter resolution | 
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| 4. Indian Remote Sensing Satellite Image (IRS) - 5 meter resolution | 
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| 5. Scanned Aerial Photo - 2 meter resolution | 
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