The octonions were discovered in 1843 by John T. Graves, inspired by his friend W. R. Hamilton's discovery of quaternions. Graves called his discovery octaves, and
mentioned them in a letter to Hamilton dated 16 December 1843. He first
published his result slightly later than Arthur Cayley's article. The octonions were discovered independently by Cayley and are sometimes referred to as Cayley numbers or the Cayley algebra.
Hamilton described the early history of Graves' discovery.
http://math.ucr.edu/home/baez/octonions/strangest.pdf
