arsis
noun
ar·sis ˈär-səs 
plural arses ˈär-ˌsēz
1
a
: the lighter or shorter part of a poetic foot especially in quantitative verse
b
: the accented or longer part of a poetic foot especially in accentual verse
2
: the unaccented part of a musical measure compare thesis
Word History
Etymology
Middle English, "raising of the voice," borrowed from Latin, "raising of the voice on an emphatic syllable," borrowed from Greek ársis "lifting, raising, raising of the foot to mark an unstressed beat (in music and prosody, opposed to thésis thesis), from ar- (contracted from *awer-), base of aeírein "to lift, raise" (also, "to join, attach, harness") + -sis -sis — more at aorta
First Known Use
14th century, in the meaning defined at sense 1a