[1] : The Collaborative International Dictionary of English v.0.48

arsis \ar"sis\ aumr"si^s, n. l. arsis, gr. 'a`rsis a
   raising or lifting, an elevation of the voice, fr. a'i`rein
   to raise or lift up. its ordinary use is the result of am
   early misapprehension; originally and properly it denotes the
   lifting of the hand in beating time, and hence the unaccented
   part of the rhythm.
   1. pros.
      a that part of a foot where the ictus is put, or which
          is distinguished from the rest known as the thesis
          of the foot by a greater stress of voice. --hermann.
      b that elevation of voice now called metrical
          accentuation, or the rhythmic accent.
          1913 webster

   note: it is uncertain whether the arsis originally consisted
         in a higher musical tone, greater volume, or longer
         duration of sound, or in all combined.
         1913 webster

   2. mus. the elevation of the hand, or that part of the bar
      at which it is raised, in beating time; the weak or
      unaccented part of the bar; -- opposed to thesis.
      --moore.
      1913 webster
see also:
metrical accentuation thesis 

Accentuation >> A Words
Accentuation, definition of term: Accentuation
accentuation_pag1.html

Metrical >> M Words
Metrical, definition of term: Metrical
metrical_pag1.html

Metrical Foot >> M Words
Metrical Foot, definition of term: Metrical Foot
metrical+foot_pag1.html