The term really means amulet, from an ancient Greek word which means "to protect." The word "phylacteries" to describe tefillin comes from the New Testament, in Matthew- it is not in the Septuagint at the part of the Bible where tefillin (or as they are called, totafot) are mentioned). The word then leaked into popular usage until Jews were basically stuck with it. It's easy to see why tefillin would seem like amulets- parchment in a leather box and worn- but it's not a particularly accurate definition in terms of how they are mostly used in Jewish ritual.