The octuple whole note (also called a maxima, duplex longa, or larga) is a note that has a value of 8 times a whole note, which is where it gets its name. In the time signature it has a value of 32 beats or 8 measures.
References
Apel, Willi. 1961. The Notation of Polyphonic Music 900–1600, fifth edition, revised and with commentary. The Medieval Academy of America Publication no. 38. Cambridge, Massachusetts: The Medieval Academy of America.
Johannes Verulus de Anagnia. 1977. Liber de musica Iohannis Vetuli de Anagnia, edited by Frederick Hammond. Corpus Scriptorum de Musica 27, 26–97. Neuhausen-Stuttgart: American Institute of Musicology.
Morehen, John, and Richard Rastall. 2001. "Note Values". The New Grove Dictionary of Music and Musicians, second edition, edited by Stanley Sadie and John Tyrrell. London: Macmillan Publishers.
Stoessel, Jason. 2009. "The Interpretation of Unusual Mensuration Signs in the Notation of the Ars subtilior". In A late Medieval Songbook and its Context: New Perspectives on the Chantilly Codex (Bibliothèque du Château de Chantilly, Ms. 564), edited by Yolanda Plumley and Anne Stone, 179–202. Turnhout: Brepols.