Definition and background:
The earliest form of musical notation from the two signs of Greek prosody (written text to be performed) indicating stress, pitch, length of syllables in the text. The acutus indicates a rising inflection of the voice.
For more information on the music term "Acutus" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary