A “grab shot” is a spontaneous capture of a scene or moment. Usually grab shots are far from polished. They may be at an angle, slightly out of focus, composed less than ideally or display other qualities of a rushed or incompletely thought through image.
There is nothing wrong with a grab shot to document a fleeting moment. A grab shot can have a negative connotation if the photo being displayed is being hyped or sold as well conceived or planned capture. As a photographer setting viewer expectation through description or title is key to managing viewer perception.
An example “grab shot” although humorously playing on the term can be seen in the following blog post “Grab Shot”.