Final answer:
Koilocytes are distinctive cells that are indicative of an HPV infection, identified by their abnormal nuclei and the clear halo around them, used as a diagnostic feature in Pap smear tests.
Step-by-step explanation:
Koilocytes are characteristic of human papillomavirus (HPV) infection. They are cells that show alterations such as enlarged nuclei and hyperchromasia, which are darkly stained nuclei typical of HPV-infected koilocytes. A koilocyte, as depicted in educational resources like Figure 23.19, exhibits a high ratio of nuclear material to cytoplasm and a distinct clear appearance around the nucleus, known as a perinuclear halo. These changes in the cervical cells are indicative of an HPV infection, differentiating it from other sexually transmitted infections (STIs) such as Trichomoniasis, Genital herpes, or Chlamydia. HPV's association with cellular changes in the cervix is also used as a diagnostic feature in Pap smear cytology tests.