Cervical intraepithelial neoplasia (CIN) refers to the presence of abnormal cells on the surface of the cervix that could potentially progress to cervical cancer if left untreated. It is typically detected through routine Pap smears and is classified into three grades based on the extent of abnormal cell growth: CIN 1, CIN 2, and CIN 3, with CIN 3 being the most severe form before invasive cancer develops.