The Torah says that one is not permitted to go to mikvah with a chatzitzah on one’s body. The definition of chatzitzah m’doraysa (see Sukkah 6) is something which covers most of the body (rubo) and which one is makpid on, which one would prefer removed. If a chatzitzah covers only part of the body and one is makpid on it, or it covers most of the body but one is not makpid (i.e. it fulfills only on of the two conditions of the Torah’s definition of chatzitzah), it is a chatzitzah m’derabbanan – one is Rabbinically prohibited from going to mikvah until such a chatzitzah is removed. The Rama writes that minhag yisrael is to be stringent about all chatzitzos and even remove that which is a miyut v’aino makpid, something that covers only a small area and which a person does not care about. For this reason women are very careful to remove anything, no matter how small, that would block contact between the mikvah water and their body during tevilah.
The gemara (Zevhacim 19) questions whether a kohein can wear tefillin shel yad on his arm while wearing bigdei kehunah and doing avodah. Rashi interprets the gemara’s dilemma as a question of whether tefillin constitutes yitur begadim, adding an extra garment to bigdei kehunah, which is prohibited. Tosfos, however, understands the question of the gemara as one of chatzitzah. The Torah commands that bigdei kehunah be worn directly on the body with no separation – do tefillin constitute a separation which violates this law? The gemara concludes that in fact tefillin shel yad may not be worn by a kohein.
The Ohr Samaiach (Issurei Biya ch. 4) is troubled by the gemara’s conclusion. With respect to mikvah the Torah defines chatzitzah as something which covers most of the body and which one would want removed – anything smaller or less obtrusive is not prohibited. Yet, with respect to bigdei kehunah the gemara assumes that even tefillin is a chatzitzah, even though tefillin is not a separation that covers most of the body. On what basis do we apply different standards to the definition of chatzitzah in these two different cases?