The posterior fat pad sign is the visualization of a lucent crescent of fat located in the olecranon fossa on a true lateral view of an elbow joint with the elbow flexed at a right angle indicating an elbow joint effusion. Normally, the posterior fat pad will not be seen in this view ref.
In the setting of trauma, it suggests an occult non-displaced fracture. In children, it most often indicates a supracondylar fracture, other common fractures are those of the proximal ulna, lateral condyle, and radial neck 2. In adults, look for a radial head fracture ref.