Dictionary:

arsis

  (är'sĭs)

1. 1. The short or unaccented part of a metrical foot, especially in quantitative verse.
   2. The accented or long part of a metrical foot, especially in accentual verse.
2. Music. The upbeat or unaccented part of a measure.

[Middle English, raising of the voice, from Late Latin, raising of the voice, accented part of a metrical foot, from Greek, raising of the foot (marking the upbeat), the unaccented part of a metrical foot, from aeirein, to lift.]

arsis, in metric, the up (or weak) beat. The metrical foot (see METRE, GREEK I) was divided between arsis (sometimes known as anacrusis) and thesis, the down (strong) beat. The terms are often confused and used in the opposite of these (original) senses.

The accented or longer part of a poetic foot; the point where an ictus is put.