In music and prosody, arsis and thesis refer to the stronger and weaker parts of a musical measure or poetic foot. Arsis and thesis were the raising and lowering of the foot in beating of time, or the raising and lowering of the voice in pitch or stress. Accordingly, in music and in Greek scansion arsis is an unaccented note (upbeat), but in Latin and modern poetry it is the stressed syllable (ictus).
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Latin and English poetry
In Latin (and Greek) dactylic hexameter, the strong part of a foot is the first syllable — always long — and the weak part is what comes after — two short syllables (dactyl: long—short—short) or one long syllable (spondee: long—long). Because Classical poetry was not based on stress, the arsis is often not stressed; only consistent length distinguishes it.
Of arms and a man I sing, who first from the shores of Troy... — Aeneid 1.1
In English, poetry is based on stress, and therefore arsis and thesis refer to the accented and unaccented parts of a foot.
Etymology
Ancient Greek ἄρσις ársis "lifting, removal, raising of foot in beating of time", from αἴρω aírō or ἀείρω aeírō "I lift". The i in aírō is a form of the present tense suffix y, which switched places with the r by metathesis.
Ancient Greek θέσις thésis "setting, placing, composition", from τίθημι títhēmi (from root θε/θη, the/thē, with reduplication) "I put, set, place".