The dark black area covers part of the definition of frozen shoulder. Based on medical knowledge, the complete sentence reads:

"The term 'frozen shoulder' is a well-defined clinical condition characterized by progressive pain and stiffness of the shoulder joint, usually resolves spontaneously after about 18 months."

The hidden/obscured words under the dark black area are most likely "clinical condition" (completing "well-defined clinical condition") and "shoulder joint" (completing "stiffness of the shoulder joint").

This matches the standard medical definition of frozen shoulder (adhesive capsulitis): a self-limiting condition of the glenohumeral joint marked by painful, progressive restriction of movement that typically resolves on its own within 18-24 months.