The Bedford Glossary of Critical and Literary Terms - Ross Murfin 2018

Arsis

Arsis: In Greek poetry, the unstressed syllable in a foot of verse (as opposed to the thesis, or stressed syllable). In later Latin usage, however, the terms became reversed, with arsis referring to the stressed syllable and thesis referring to the unstressed syllable. According to Greek usage, the arsis of the word arsis falls on the second (the unstressed) syllable, whereas in Latin usage, the arsis of the word arsis falls on the first (the stressed) syllable. In prosody (the study of versification), both usages persist, though the Latin usage is more common.