Biochemical recurrence refers to the reappearance of prostate-specific antigen (PSA) levels in the blood after primary treatment for prostate cancer, indicating that the cancer may be returning or progressing. This recurrence is often detected before any physical signs or symptoms of cancer are present, making regular monitoring of PSA levels an important aspect of post-treatment care for prostate cancer patients. Biochemical recurrence can occur after surgery, radiation therapy, or other treatments, and may require further treatment or monitoring to manage.