Paraphimosis occurs when the foreskin is retracted behind the glans penis and cannot be returned to its normal position. This entrapment causes constriction at the corona, leading to venous congestion, swelling, and potential ischemia of the glans. Prompt reduction of the foreskin is critical to restore normal blood flow and prevent tissue necrosis. It is a surgical emergency distinguishing it from phimosis, where the foreskin cannot be retracted at all. Therefore, the defining feature of paraphimosis is foreskin retracted and unable to be reduced.

Reference: Campbell-Walsh Urology, Wein, 12th Edition.