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.
Variation and Deviation. Variation (or declination) is caused by the fact that the magnetic north pole is not located at the same spot as the true North pole. Nothing we can do about that.. Some other definitions: lines of equal variation are called isogonic and were the variation is zero it is called an agonic line.