A Hill-Sachs lesion is defined as a coritcal depression of the posterolateral head of the humerus, according to Wikipedia. This condition usually comes from recurrent anterior shoulder dislocations and generally results in compression fractures on the posterolateral humeral head. Because of this, it is commonly associated with an additional type of lesion, known as a Bankart lesion.
A Hill-Sachs lesion is acutely painful at the time of dislocation, and unfortunately, may promote future dislocation because of the lever-like effect it has during external rotation that effectively causes instability to the shoulder joint. This type of defect often requires a CT or MRI scan to be detected, as it can be difficult to see on an X-ray. Often, the bony defect itself does not require treatment, but surgical repair is often required because of the damage it has on surrounding tissues. According to Radiopaedia.org, larger defects are more commonly repaired with the use of bone graphs or the placement of soft tissue within the defect.
The lesion is named after two American radiologists, Harold Arthur Hill and Maurice David Sachs. They first discovered it in 1940 when they published a report of 119 cases of shoulder dislocations. Wikipedia asserts that prior to Hill and Sach's paper, the fracture was understood to be a sign of shoulder dislocation, but the precise mechanism was not certain.