Although tefillin is technically the plural form (the singular being tefillah), it is loosely used as a singular as well. The arm-tefillin, or shel yad, is placed on the upper arm, and the strap wrapped around the arm/hand, hand and fingers; while the head-tefillin, or shel rosh, is placed above the forehead.
Definition of declination. 1 : angular distance north or south from the celestial equator measured along a great circle passing through the celestial poles. 2 : a turning aside or swerving. 3 : deterioration moral declination. 4 : a bending downward : inclination.
Magnetic declination, sometimes called magnetic variation, is the angle between magnetic north and true north. Declination is positive east of true north and negative when west. Magnetic declination changes over time and with location.