What is the ruling on wearing a cap on the head instead of a hijab?

Any garment that meets the conditions of the Islamic hijab (definition) is a legitimate hijab, regardless of its name. As for the "taqiyah" (a type of head covering), if it fulfills the conditions, then it is a hijab. Otherwise, the one who wears it might fall under the description of "women who are dressed yet naked, their heads like the humps of leaning camels," mentioned in the hadith (definition).