It could be an image, a sound, a video or a model.
Artist's impressions are often created to represent concepts and objects that cannot be seen by the naked eye; that are very big, very small, in the past, in the future, fictional, or otherwise abstract.
For example, in architecture, artists' impressions are used to showcase the design of planned buildings and associated landscape.
[1] Artists' impressions are particularly prominent in space art.
[2] Artist's depictions of prehistoric life are known as paleoart.