The Collaborative International Dictionary of English v.0.48:

Phylactery \Phy*lac"ter*y\, n.; pl. Phylacteries. [OE.
   filateri, OF. filatire, filatiere, F. phylact[`e]re, L.
   phylacterium, Gr. fylakth`rion, fr. fylakth`r a watcher,
   guard, fyla`ssein to watch, guard. Cf. Philatory.]
   1. Any charm or amulet worn as a preservative from danger or
      disease.
      [1913 Webster]

   2. A small square box, made either of parchment or of black
      calfskin, containing slips of parchment or vellum on which
      are written the scriptural passages Exodus xiii. 2-10, and
      11-17, Deut. vi. 4-9, 13-22. They are worn by Jews on the
      head and left arm, on week-day mornings, during the time
      of prayer. --Schaff-Herzog Encyc.
      [1913 Webster]

   3. Among the primitive Christians, a case in which the relics
      of the dead were inclosed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

phylactery
    n 1: (Judaism) either of two small leather cases containing
         texts from the Hebrew Scriptures (known collectively as
         tefillin); traditionally worn (on the forehead and the left
         arm) by Jewish men during morning prayer [syn:
         phylactery, tefillin]