Tefillin (Askhenazic:  /ˈtfɪlɨn/; Israeli Hebrew: [tfiˈlin], תפילין) also called phylacteries (/fɪˈlæktəriːz/ from Ancient Greek phylacterion, form of phylássein, φυλάσσειν meaning "to guard, protect") are a set of small black leather boxes containing scrolls of parchment inscribed with verses from the Torah, which are worn by observant Jews during weekday morning prayers. Although "tefillin" is technically the plural form (the singular being "tefillah"), it is loosely used as a singular as well. The hand-tefillin, or shel yad, is placed on the upper arm, and the strap wrapped around the arm, hand and fingers; while the head-tefillin, or shel rosh, is placed above the forehead. The Torah commands that they should be worn to serve as a "sign" and "remembrance" that God brought the children of Israel out of Egypt.

The scriptural texts for tefillin are obscure in literal meaning. For example, the verse in Deut. 11:18 does not designate what specifically to "bind upon your arm," and the definition of totafot is not obvious. It is the Talmud, the authoritative oral tradition for Rabbinic Judaism, which explains what are to be bound to the body and the form of tefillin.