ar·sis 
(ärsĭs)
(ärsĭs)n. pl. ar·ses (-sēz′)
1.
a. The short or unaccented part of a metrical foot, especially in quantitative verse.
b. 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; see wer-1 in the Appendix of Indo-European roots.]